The slang word "mega" means "big" and it comes from the Greek word megas (μέγας), which means "great." Put "mega" in front of anything and it's instantly much larger than what you started with. As the Collins Dictionary rather stodgily puts it, "Young people sometimes use 'mega' in front of nouns in order to emphasize that the thing they are talking about is very good, very large, or very impressive."
As luck would have it, this mega fabulous word "mega" is also used in German!
Und die Clubs sind natürlich megawichtig [umgangssprachlich, sehr wichtig].
And the clubs are, of course, mega [slang, very] important.Play Caption
Am zweiten Jahrestag der Megapleite sind in Frankfurt erneut Menschen auf die Straße gegangen.
On the second anniversary of the mega-crash, the people in Frankfurt took to the streets again.
Caption 6, Finanzkrise - Die Lehman-PleitePlay Caption
You probably won't find "mega" in the works of William Shakespeare, nor will you impress your academic friends by slipping the word into the conversation, but a bit of slang in your spoken German might make you sound just that much more like a native speaker. Read this hilarious article about the 1980s origins of the word as English slang and find some more examples of "mega" on Yabla German to see how it is used in a real-world context.