The time signature is indicated at the beginning of the measure using two numerals:
The upper number indicates the number of beats in the measure, 3 in this case.
The lower number indicates the note that will occupy one beat. In the next table you see the relationship between notes and numbers:
Therefore the time signature indicates that the measure has 3 times and that each beat is occupied by a quarter note. Notice that the 4 symbolizes the quarter note because it's 1/4 of the whole note.
We frequently use the symbols and to indicate the 4/4 () and 2/2 ().
See Reading Music