The verb erklären has two very different meanings, as does its noun form die Erklärung. One word having multiple meanings seems to happen more often in German than in English, if only for the fact that the German language has far fewer words than English. With fewer words in a language, it's more likely that a single word will have multiple meanings than in a language with a larger vocabulary!

The most common meaning of erklären is "to explain":

Kann jemand erklären, was ein Binnenhafen ist?

Can someone explain what an inland port is?

Caption 27, Bundesländer und ihre Rezepte: Nordrhein-Westfalen

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Wir erklären euch, wie man in Deutschland Ostern feiert.

We'll explain to you how you celebrate Easter in Germany.

Caption 3, Cettina und Sabine: Ostern

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And the same applies to its noun form, die Erklärung:

Vielleicht gibt's eine einfache Erklärung für seine Angst.

Perhaps there's a simple explanation for his fear.

Caption 62, Großstadtrevier: Von Monstern und Mördern

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Es braucht Erklärungen, um die Brutalität und Banalität des Unrechts zu verstehen.

Explanations are required to grasp the brutality and banality of this injustice.

Caption 19, DDR zum Anfassen: Ganz tief im Westen

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However, the second most common meaning of the verb erklären (and its noun equivalent die Erklärung) is as follows:

Kein Problem mehr für die Bewohner vor Ort, erklären sie stolz.

No problem anymore for the local residents, they declare proudly.

Caption 20, 30 Jahre: Menschen für Menschen

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Ich erkläre hiermit, dass Sinus einem Tierheim übergeben wird.

I hereby declare that Sinus will be handed over to a shelter.

Caption 70, Yabla Gerichtshof: Das verzogene Hündchen

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Note that the noun die Erklärung, when used with the meaning "declaration," is often paired with another noun as the latter half of a compound noun:

Jetzt also die Liebeserklärung an das Meer.

So now, the declaration of love for the sea.

Caption 26, Herbert Grönemeyer: stellt sein neues Album vor

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Eine entsprechende Absichtserklärung zwischen der People's Bank of China und der Deutschen Bundesbank wurde am vergangenen Freitag in Berlin unterzeichnet.

A corresponding declaration of intent between the People's Bank of China and the German Federal Bank was signed last Friday in Berlin.

Captions 3-4, Frankfurt wird Handelszentrum: für die chinesische Währung Yuan

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There are a number of German verbs nouns for "declare," such as the rather obvious deklarieren and the less obvious verkünden. Some nouns using die Erklärung as "to declare," such as die Kriegserklärung, also have words with similar meanings, such as die Kampfansage. Both of these words mean "declaration of war," something we've all seen far too much of in the last several years.

Further Learning
Luckily, in real-world usage, it's usually quite easy to tell by the context in which the words are used if the meaning "to explain" (explanation) or the meaning "to declare" (declaration) is intended. For details of the verb's conjugation, see this video about erklären on Yabla German.

Most nouns that are masculine and end in -e in their nominative singular form are called weak nouns, or schwache Substantive. They are often nouns that refer to nationalities, animals, or professions, such as der Schwede, der Löwe or der Psychologe. Since most German nouns ending in -e are feminine, these exceptions are easy to recognize. What sets them apart from other nouns is the fact that in all cases except for nominative, they end in -en. It is obvious that nouns ending in -e will end in -en in plural cases, but this applies to the singular case too!

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Here's examples of the weak noun der Junge with the -en ending in different singular cases, starting with dative:
 

Der Zauberer sah zu dem zitternden Jungen hinab.

The Sorcerer looked down at the trembling Boy.

Caption 80, Märchen - Sagenhaft - Der Zauberlehrling

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And again in the singular accusative case: 
 

"Schiffsverkehr"... für einen Jungen aus dem Pott klingt das nur bedingt naheliegend.

"Ship traffic"... for a boy from the "Pit" [Ruhr region] that sounds only conditionally obvious.

Caption 9, Herbert Grönemeyer - stellt sein neues Album vor

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And finally in the singular genitive case: 
 

Der Fisch wurde von der Mutter des kleinen Jungen gekauft.

The fish was bought by the mother of the little boy.

Caption 65, Märchen - Sagenhaft - Der standhafte Zinnsoldat

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There are also some weak nouns that end in letters other than -e, but more about them later! 

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Further Learning
Go to this page to practice your weak noun endings, and visit Yabla German to find more examples of weak masculine nouns in practice.