it simply means "Im completely/totally OK with this"
Cool, here refers to be ok with a certain situation. for example: If u come to my party and I feel ur a little bit uncomfortable, I may ask "Could I get u anything to drink?" and u could say "No thanks. I'm cool" it means no thanks, Im ok...
"What do u say we goto Dalian this weekend?"
"Im cool" (Its ok, I don't mind, Im ok with this, yeah sure)