See the thumbs in the top right hand side of this page. Click one of them and you will have done an 'i rate it'.
Bob 'Check out the cans on the chick over there Jack'.
Jack 'Yeah Bob alrigh, I rate it'. and gives Bob the thumbs up as he replies.
a shortened version of rating something, usually the looks of a girl. It started from somebody asking "what would you rate that girl, 1-10". ppl then shortened it to i rate it, meaning i would take the time to rate that because she is hot enough to catch someones attention. If somebody walks by unoticed because she is not hot
, she does not deserve to be rated, so nobody would "rate it"
Therefore: I rate it is a way to let somebody know that a particular girl is hot
* A Hot Girl Walks By *
man 1: I rate it
man 2: yeah i agree shes damn sexy