You're music needs to fit in four blocks. ( or 8 or 16 or ...)
If you go over the last bar, you create a cap. ( or if you don't fill all 4 blocks )
Hope that makes sense.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Marcus Brigstocke
That is a question about note-length
In a DAW you have the piano-roll divided in pieces. The size of these pieces depends on the time-signature
The precision allowing You to insert notes is called the Quantization or Q
In default lmms you have 4/4 time-signature and 1/16 Q
That means that in a bar, you have 4 beats and in a beat you have 4 ticks
If your note-length is non-fit-able with these std, then you get 'gabs' between notes or note segments!
I explain a lot more about using time-signatures, and Q values in the video here: https://youtu.be/24iarkQyQ_8