Balancing the lengths of bars

"System" is a fancy musical term for "line". But a bit more specific. Consider piano music - there are actually two staves in each "line" of music. So, a "system" means, a line consisting of *both* of those staves. A page of piano sheet music might consist of 8 "staves, but that's really 4 systems of two staves each. So in MuseScore, there is nothing scary about "system".

With that out of the way, I don't actually recommend messing with that particular option (the one to fix the number of measures per system). For one thing, it's inflexible - no way to allow for a pickup on the first line, or an extra mesasure of two on lines thaty have 1st and 2nd endings, etc. Also, lots of weird things can happen if you forget you have that option turned on and then starting messing with other aspects of page formatting - like the "stretrch" settings.

So for short pieces, I really recommend just inserting line breaks manually - click barline, hit Enter. For longer pieces where that might be more work than you want, try Plugins / Break Every X Measures. This will simply insert those breaks for you, achieving the same end result as the Fix option, but more "transparently" (you can actually see the line breaks that were isnerted, and delete them or add new ones as you see fit). This works on a selection, so if you want five measures on the first line (to include the pickup) but four on the rest, you can insert that first line break manually, then select the rest of the measures and run the plugin to insert breaks every 4 bars.