Each horizontal line represents a string (the lowest string being the bottom line, and the highest string being the top line) and numbers placed on strings indicate on which fret you have to place your fingers.
On this exemple, the number 5 is placed on the bottom line, you have to place your finger on the 5th fret and play the E string (= play an A on the E string).
If the number is a 0, you have to play the open string.
As an example, the same A can also be represented by the number 0 on the A string.