Each generation has a different perception — view, idea or understanding — of what is cool. You wouldn't want to walk around in the paisley patterns of the 1960s or the big hair of the 1980s today!
Based on the Latin root cipere "to grasp," perception refers to the way you take in the world through your senses. Have you ever thought a stair was bigger than it really was so your step was too heavy? Your depth perception was off. The noun also means the opinions and beliefs you've formed about something. Your perception of your room after you've cleaned it is different than your parents' — you see clean and they probably don't.