The notefinder helps you to find out which note is played on which string and on which fret. ‘0’ means that you play the open string (plucking without fretting), ‘1’ signals that the string is fretted on the first fret, etc.
The strings (from thick to thin) are called: E, A, D, G. Pictured, the E-string is the lowest.
By using the notefinder you may e.g. find out that the note D shown on the staff’s middle line can be played in three different ways - as open D-string, on the fifth fret of the A-string and on the tenth fret of the E-String. Which one of the three possibilities you actually choose depends on the position you are playing. If you play the third position for example, you play the D on the fifth fret.
If the notes look unfamiliar to you and quite different from what you have learned at school this will probably be so due to the fact that we do not use the treble clef but the bass clef. Of course, bass guitar notes are always notated in the bass clef.