The word sei is something that can be a bit confusing when it appears. Although it is rare and more often used in written German, sei does comes up on Yabla German every once in a while.
It is, for one, the informal second person command form of the verb sein ("to be").
Wir haben Fieber, komm sei dabei
We have the fever, come be involved
Caption 11, Christina Stürmer - FieberPlay Caption
Sei ruhig, Findus, ich bin ja noch gar nicht aufgestanden.
Be quiet, Findus, I indeed haven't gotten up yet at all.
But sei appears in other contexts as well. The phrase es sei denn can be translated as "unless":
Man hat uns erzählt, sie läge bis zum heutigen Tag dort,
Someone has told us [it is said] that it is there to this very day,
es sei denn, es hat sie jemand gegessen.
unless someone has eaten it.
Captions 93-94, Märchen - Sagenhaft - Die Prinzessin auf der ErbsePlay Caption
Sei is particularly used when something is or was reported or thought to be true, but isn't proven. However, it is used most often in written German, or narration and reporting.
Also hat der Papst die Armbrust verboten
So the Pope forbade [the use of] the crossbow
und hat gesagt, es sei ein Werkzeug des Teufels.
and said it is a tool of the devil.
Captions 30-31, Die Armbrust - im MittelalterPlay Caption
Nein, der unbekannte Verkehrsteilnehmer hatte nur irrtümlich angenommen, die Parkuhr sei beschädigt...
No, the unknown motorist had just mistakenly assumed the parking meter was damaged...
Die böse Königin glaubte, Schneewittchen sei tot...
The evil Queen believed Snow White to be dead...
Caption 51, Märchen - Sagenhaft - SchneewittchenPlay Caption
Do you know all of the conjugations of sein in the imperative? If not, review them now with the table on this page. When you encounter sei in its other contexts, remember that it essentially communicates the subjunctive and what is being said may not be true at all!