To be "wet behind the ears" is to be inexperienced, naive, or immature. This phrase was more commonly used before "newbie" became a popular term. For example:

"These new recruits are really wet behind the ears."
or
"I would've asked him out to dinner if he weren't so wet behind the ears."

The origin of this phrase, like so many other American idioms, comes from the farm. When a baby calf is born, its body is covered entirely in birth fluids. The animal dries quickly, save for the spot just behind the backs of its ears. You can tell which calves are newborns by the damp spots behind their ears.