system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, what is a bar in music?

So what is a musical bar? I looked this up on Google but the first five links weren't clear.  I'm very unintelligent, so don't worry about speaking down to me.
gpt
A bar is another word for a "measure," which is just a regular pattern of beats in music.  Most commonly 4 beats, though 2, 3, or other numbers are common.

It's easiest to think of bars in modern rock and pop music, because it's usually distinctive and repeated.  Think of the beat to "We Will Rock You."  Boom Boom Clap (Rest), Boom Boom Clap (Rest), Boom Boom Clap (Rest).  Each of those "bars" is made up of four beats - two booms, one clap, and then a rest before the next beat starts.  Each of those repetitions is one bar or measure of the song.

Rock music usually has sections like verses, chorus, etc. that are 8, 12, or 16 bars, though there are plenty of exceptions to this rule.  "We Will Rock You" has a fairly classic 8-bar structure.  8 bars of verse (Buddy you're a boy make a big noise...) and then 8 bars of chorus (We will, we will rock you...).  Give it a try yourself counting along if you like!  

And it doesn't matter much to the average listener, but it can be helpful music theory if you're writing your own songs or trying to recognize patterns in music.