To ascend is to move or travel upwards. You've heard about how we ascend to Heaven after we die? Well, that's because Heaven is above us, so we get to go up!
Riding the escalators in the department store is one way to amuse yourself while your mom does the shopping. You can ascend all the way to 8 (Women's Lingerie) and then descend again, all the way back to the ground floor. Both of those words come from the Latin verb scandere, which means "to climb." It's the prefixes that make all the difference. The a- of ascend makes the word mean "climb up," while the de- of its antonym makes descend mean "climb down."