Bar Rests

They are there when you empty the bar. Any bar with no notes is automatically filled with either a full measure rest that looks like semibreve (whole note) rest but centred, or a combination of rests that add up to the full duration of the measure. To convert the latter into the former select the whole measure and press delete.

Try looking for them on this handbook page: https://musescore.org/en/handbook/3/measure-rests