It is generally used as an expression of appreciation for an attractive woman, or as an indicator of how do-able she appears to you.
It refers to the number of times you would be willing to engage in anal intercourse with said woman.
It may occasionally be used, usually with a smilie, to denote the exact opposite, signifying that there is, in fact, no way you would be willing to take a drive up the old dirt road with a particulary unattractive woman.
This expression is only known to be used by the inhabitants of the beeb (WERA BBS).
The multiplier 2 may sometimes be increased to reflect a particularly porkable female (3XITB, 4XITB, etc.) Records indicate that no female has ever exceeded the rating of 7XITB.
No multiplier lesser than 2 should be employed when used to signify a delicious piece of eye-candy.
She is most definitely a 2xITB for me too.
2xITB for sure.