There are 6 tab lines representing 6 strings on the guitar. (The 1st string is the thinnest, and the 6th string is the thickest) They are as follows:

E----------------------------- String 1
B----------------------------- String 2
G----------------------------- String 3
D----------------------------- String 4
A----------------------------- String 5
E----------------------------- String 6

When a number is placed on one of the lines, it indicates the fret location of note for example,

E----------------------------- String 1
B------4---------------------- String 2
G----------------------------- String 3
D----------------------------- String 4
A----------------------------- String 5
E----------------------------- String 6

(This means to play the 4th Fret on the 2nd string)

E----------------------------- String 1
B----------------------------- String 2
G----------------------------- String 3
D----------------------------- String 4
A------0---------------------- String 5
E----------------------------- String 6

(This indicates an open 5th string)

E---1------------------------- String 1
B-----------8----------------- String 2
G-------7--------------------- String 3
D---------------2------------- String 4
A----------------------------- String 5
E------------------------12--- String 6

This indicates:
1st Fret, 1st string then
7th Fret, 3rd string, then
8th Fret, 2nd string, then
2nd Fret, 4th string, then
12th Fret, 6th string.