"U" is for......

"Use it or lose it."

This one is saying that not using a skill may cause us to lose it.

For instance:
A man knows how to play guitar.
But, he puts his guitar down one day. And then, he doesn't pick it back up for 20 years.
When he does pick it back up to play, he has found that he can't do it as effortlessly as he was once able to. He has forgotten chords; he can't play in time.
Because he has not used his skill in so long, he has lost some of it.

And so, if we have a skill, we must always use it so that we can always keep it.

That is what this proverb means to me.