The time signature is indicated by two numbers at the beginning of the bar:
The upper number indicates the number of beats in the bar, in this case 3.
The lower number indicates the note that will occupy one beat. The next table shows the relationship between notes and numbers:
Therefore the time signature indicates that the measure has 3 beats and that each beat is occupied by a quarter note. The number 4 symbolises the quarter note, because it's 1/4 of the whole note.
We often use the symbols to indicate and for .