Repeats are a sign that indicates a section should be repeated. An 'End Repeat' sign would be placed at the end of a section, and that means you should play that passage again. If there is a 'Start Repeat' sign, you go back to that sign or if there isn't, you just go back to the beginning.
Just before the 'End Repeat' sign, there may be a '1st ending' bracket above one or two (or even more) bars. This means on the first time you play that phrase, you play the 1st ending and then repeat to either the 'Start Repeat' sign or the beginning. Once you arrive to the first time bar again, you will move onto the second time bar instead. This will always be found after the repeat sign.
In this opening to 'Can't Take My Eyes Off of You' by Bob Crewe and Bob Gaudio, there is a start repeat sign at the start of bar 9, and the end repeat sign at the end of bar 24. There is the first time ending over bars 23 and 24, and the second time ending over bars 25 and 26.Â
In bar 9, there is a 'second time only' marking. This means that when playing past bar 9, you only play the section the second time you come to it. This is until there is a marking that says 'play both times' or something similar.
When playing this song, you would start at the beginning, and play all the way to the end of bar 24, where you would go back to bar 9, then play until the end of bar 22, where you would go to bar 25.
Repeats are very useful because they save paper when writing music, and can make sight reading a lot less intimidating. They are an easy way of showing where to go on certain sections.