When you do a division problem and the quotient (answer) ends up being with a number that just keeps repeating say like when you divide
20 ÷ 9 = 2.22222222222
In this case you don't have to write all these 2s, you just put a line on top of the second 2 after the decimal to show that it repeating.
_
2.22
That line is a repeating bar.
Hope this helps. :)