In general, banjo strings are numbered from 1 to 5. The highest-pitched string is the 1st string, and the lowest-pitched string is the 5th string.
1st String - The highest string of a banjo, typically referred to as the "first" or "high" string.
2nd String - The next-highest string, often called the "second" or "middle" string.
3rd String - The middle string of the banjo, sometimes referred to as the "third" or "thumb" string (as it's often played with the thumb).
4th String - The second-lowest string, also known as the "fourth" or "tenor" string.
5th String - The lowest string of a banjo, often called the "fifth" or "bass" string.
The strings on a banjo are generally tuned to an open chord, such as G, C, or D, but can be tuned to various other tunings depending on the style of music being played.