crotchet rest, sometimes called 1/4 note rest. If you are in crotchet time it is one beat rest, if you are in minim it is 1/2 or if you are in quaver time it is 2 beats.
Music is with the breathing & vocal cord, so when in a opera all are given their notes of their part of music with pause & rests. This shows when he is to rest.
Think musical notation.
It is a quarter beat rest in 4:4 time that means that there are only 3 other played notes on the bar