They are numbered from 1 to 6 - the first being the thinnest and the 6th being the thickest.
Tablature is a six-line staff that represents the guitar fingerboard.
By placing a number on the appropriate line, the string and fret of any note can be indicated. The number 0 represents an open string. If they are written out like this they are to be played one at a time. Reading grids is about the same. The 6 lines represent the strings of the guitar.