"Setting a bar" or "setting the bar" means making a standard or level that other people or things will have to meet.
For example, if you are older than your other siblings, and you get great grades in school, your parents will expect your other brothers and sisters to do do well in school as well. You've "set the bar" for them. Specifically, you've "set a high bar" for them, because they'll have to work hard to do as well as you.
You can also "set a low bar". For example, if you're going to watch an action movie with a friend, and she says "I just hope it's better than (some really bad movie)", you can say:
You're setting the bar kind of low, don't you think?
You can set a high or low bar on purpose, by deciding and announcing your expectations:
I try to set the bar very high for my students. I expect a lot out of them, and I think that's why they turn out to be so successful.
Notice that you can "set a high bar" or "set the bar high", and the same for "set a low bar" or "set the bar low".
Someone can also "set the bar" without specifying whether it's high or low:
The first performance really set the bar for this whole competition.
And you can "raise" or "lower" the bar:
His performance has raised the bar for everyone in the league.
This expression seems to come from the sport of high jumping, in which jumpers have to try to jump over a bar set at certain heights.