It means exactly.. without the e! A far cooler way of saying it.
When speaking it aloud, it's pronounced "zack-lee"
Mark: And that is why he's a faggot.
Take the egg out---the lazy, slang, text version of exactly.
Can be used as a term of agreement, exasperation, sarcasm plus other things I don't care to type.
"This food is delicious! Glad I let you choose the restaurant."
"Man that guy sure was a numpty, xactly like you said."
MAN "I'm going out to run some errands, hon."
WIFE "How long will that take xactly, you have to drive the kids to (insert event here)."
Basically, it's means the same as "exactly", but it's actually way more expressive. Leave out the 'E' and suddenly it sounds more powerful!
Have you noticed Sky TV's zillion channels are just full of shite TV? Xactly!