Percentage is the amount of lights goes in.
So, %10 is darker than %90.
At least that's what I remember. I am not much of a tinted low-ryder kinda guy, but this should be accurate.
Remember, not all factory tints are legal for everystate. If you buy your jeep in ohio and bring it to NY you may get slapped with tickets. To be on the safe side Jeep does not tint front windows, rather than customizing to each state.
What one should do is, check DMV of your state and find out what is the % tint you can get on front windows. For some reason salvaged cop cars are almost black in the front windows, and if you buy one for like $1000 at an auction you can drive away with dark as black tints, and nobody seems to get a ticket on those.. :-#
*edit:
New York Window Tint Law
New York Tint Law Enacted: 1991
HOW DARK CAN WINDOW TINT BE IN NEW YORK?
Darkness of tint is measured by Visible Light Transmission percentage (VLT%). In New York, this percentage refers to percentage of visible light allowed in through the combination of film and the window.
Windshield= Non-reflective tint is allowed on the top 6 inches of the windshield.
Front Side Windows= Must allow more than 70% of light in.
Back Side Windows= Must allow more than 70% of light in.
Rear Window= Any darkness can be used.
Sticker Requirements= The sticker to identify legal tinting is required between the film & glass on each tinted window.
http://www.tintcenter.com/