So on the E string, 10D indicates hold down the string on the tenth fret, get a D note.
The second string is the B string. The third string is the G string; the fourth is the D string and the fifth is the A string.
The sixth string (bottom string on the diagram, and the thickest string), is also an E string. But its notes are of considerably lower frequency than the first string's. Here is a description of standard tuning, with an explanation of the notes within each octave (and of the C1, E2 notation). The frequencies of the various notes are listed here.
Notes are colored by octave. The lowest octave is black (containing notes E2 through D3; the next higher one is blue (containing notes E3 through D4); the next higher octave is red (containing notes E4 through D5), and the highest octave is in violet. It contains notes E5 through D6, although on my diagram one can only go to G5. Middle C is C4; you get it by holding down the B string on the first fret (the 1C in blue on the diagram).
Guitar music is written one octave higher than the actual pitch. So be careful if using music written for the guitar on other instruments.
