1 - word heavy, hectic. something very good.
2- also used in place of "interesting" when something isn't so good
3- okay or really? used when no other appropriate words available. Also used to mean profound when in this context.
1 - "This tune is deep. For real"
2- "What did you think of my singing?"
"Er, yeah... It was... deep"
3- "I just got mugged."
"That's deep"
Someone who niether writes nor understands poetry, will use the term "Deep" to describe a poem.
Bob: (In reference to a poem) "What do you think of the line: "You're a mouse with no whiskers"
Jake: "Yeah, dude, It's like super deep!"
Bob: "You don't get it, do you?"
Jake: "Yeah I do, It's just super deep!"