If you always forget where you put your glasses or your car keys, you can describe yourself as absentminded. An absentminded person is forgetful and often dreamy.
If you move through your days lost in thought, or daydreaming, you are probably at least a little bit absentminded. Being absentminded means you have a lack of focus or attention, and you forget small details like the fact that your reading glasses are on top of your head. The stereotype of the "absentminded professor" is a very smart, perpetually forgetful character who's oblivious to what's around him.