Dexterity is a measure of your hand-to-eye coordination, or how skillful you are at working with your hands. This attribute would be used for things such as operating a tool, painting a picture, jimmying a lock, aiming a gun or bow, performing sleight-of-handtricks , or trying to pick someone’s pocket.
Someone with a Dexterity of 10 may not have all their fingers intact, or if they do, they were recently broken.
A Dexterity of 30 belongs to someone who drops things on a regular basis, or squirts the cologne in his own eye, or hits his thumb instead of a nail with a hammer.
45 is a perfectly average Dexterity.
60 Dexterity means that the person is probably a fairly skilled artist, woodworker, jeweller, etc.
A Dexterity of 80 indicates that the person could probably either flawlessly paint miniature figurines, perform brain surgery, or shoot a flea in the eye from a hundred yards, or possibly all those things combined.
A Dexterity of 100 would be well on the borders of the human capability for hand-eye-coordination. This person would either be an Elven archer, an Android surgeon, or some weird combination of the two.