To rub something out means to remove it by rubbing. For example, you can rub out a mistake on paper with an eraser.
Rubbing something out can make people think of fixing mistakes or making things clean.
Examples:
I will rub out the pencil mark on my paper.
She rubbed out the wrong answer.
Can you rub out the drawing on the board?
I tried to rub out the mistake, but it's still there!
He rubbed out the whole word and started again.
The teacher rubbed out the wrong number and wrote the right one.