To me, this word can have 2 meanings:
1) "Epic" - as adjective - to describe something that is very very very very cool or funny or even fantastic (without needing to say the word "very" 10 times!)
"Kelly turned 19 yesterday and her dad's going to take her to the local theme park that just opened!"
"Oh wow, that sounds epic! I really want to go too!"
2) "Epic" - also as an adjective - to describe something that is very complicated or very hard to do or complete.
"Oh, John, you know that game I bought a week ago at the game store? It's so epic, I haven't even got past the first level!"