What does No one is perfect that's why pencils have erasers mean?

Well, no one can write perfectly, right? so if you mess up, then you can fix it. If everyone's writing was perfect, then there would be no need for erasers, right? So think of the writing as a problem. You messed up, so the eraser fixes it. Sorry if you still don't get it, it's kind of hard to explain.

Meaning, that if you mess up on paper, you can erase it so the creator of this quote is trying to compare writing to life. When you mess up in life, you aren't perfect and you can't go make in time to stop yourself from doing it, but you can correct it. Just as an eraser would on paper.