Good question! Vented kites have mesh panels in their sails (kind of like nylon screen that might be in your windows), or a series of Swiss cheese-like holes. The vents intentionally let some of the wind through.
Why would you do this?
One reason is to allow a kite to fly in heavier wind without breaking.
Another is to allow sport kites to fly slower in strong winds (making them easier to control) than their standard-sail brethren. Vented kites also tend to be less responsive to gusts, making them a little easier for beginners to control in gusty winds (especially valuable with Revolution kites).
Here's a picture of a vented Revolution kite that was recently offered for sale on this forum. Note the two see-through strips on each side. If you look closely, you'll see that they're covered with black mesh.
Here's a picture of a Revolution without vents:
Here's a picture of the Flying Wings Silver Fox vented. Note the use of a matrix of holes near the wingtips: