Do you understand the difference between BUILD and DESCEND?
Imagine you are leading your lane.  The next set includes a section with 3 x 50 build.
Later, it includes 3 x 50 descend.  Will you be able to lead the lane correctly?  Let's review!

3 x 50 build

When asked to build, each 50 will be about the same speed.  Build means you typically start off with powerful speed.  As you go, your speed and power will increase to a maximum amount of speed and power when you finish at the wall.  You build on the speed you started with.

3 x 50 descend
When asked to descend, each 50 will get faster, and your time descends You are asked to swim each repetition faster than the previous.  Descend means to go down, therefore, if you timed your 50s, each one would have a faster time.

For example:

First 50 is fast...perhaps, 40 seconds
Second 50 is faster...38 seconds
Third 50 is fastest...35 seconds

WHY are these methods helpful for swimmers?

Building helps the swimmer work on speed while maintaining good stroke technique.  It also helps to perfect the finish since you should be at your maximum speed coming into the wall for the finish.

Descending times gives the swimmer more rest at the wall as the speed and intensity of each repeat increases.  Descending also teaches swimmers how to change speeds during a set.