To ascend is to move or travel upwards. You can do this literally, like when you ascend to 35,000 feet after your plane takes off, or figuratively, like when you win the lottery and ascend to a higher social class.
Riding the escalators in the department store is one way to amuse yourself while your parents shop. You can ascend all the way to the 8th floor, and then descend back to the ground floor. Both ascend and descend 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."