Before I explain what is ascending, I would like to remind you of a maths we did in childhood.
We all did maths, right? There was a kind of maths called : ascending and descending order, remember? What do we had to do in ascending order : put numbers in order, small to great. Notice that it is like a staircase. We are stepping towards higher steps(number, sorry!). It's the opposite, in the case of descending, we have to go downwards the staircase(dealing with numbers, actually!)
So,what's it? To ascend means to go upward. It can also mean to move or climb upwards - situation matters, you know.
For example :- The grand prince of the Wales ascended the horse.
Which means: The grand price of the Wales climbed the horse. (climbed upwards, right)
And to exaggerate the matter, the antonym of ascend is descend.
For example :- The grand prince of the Wales descended the horse.
Which means: The grand prince of the Wales climbed down the horse.
So, it means going downwards. Moving or also climbing downwards.