An open string note is indicated by a black letter on a white dot. To play the note G, pick the open 3rd string. (No fingers placed behind the frets)
A fretted note is indicated by a white letter on a black dot. To play the note A place the second finger of your left hand just behind the second fret of the 3rd string.
This is a quarter note.
It lasts for one beat.
There are four quarter notes in one bar of * time.
In this example the notes G and A on the 3rd string are played in quarter notes. Notice that there are four quarter notes in each bar. Play slowly and evenly, counting out loud as you play. It is also a good idea to practice naming the notes out loud as you play. Notice the numbers on the TAB indicating the open 3rd string (G) and 2nd fret of the 3rd string (A).
Here is another example using the notes G and A. Notice the chord symbols written above the music. These indicate the chords another instrument (e.g., guitar) would play to accompany the melody. Once you are confident that you can play the example evenly without stopping the beat, try playing both the melody and chords along with a metronome. Being able to swap between melody and chord playing is important for all banjo players.
Remember that you can choose to read the traditional music notation (top line of music), or the Tablature (second line).