Look at the example above. Each of the four lines indicates one of the strings, just like a fretboard diagram. The letters on the left side correspond to the notes that the open strings are tuned to. Any unusual tunings required for a song will be shown here. The top is always the thinnest string, and the bottom is always the thickest string.
The numbers represent frets. The first metal bar down from the nut is fret number one. If you see a 1 in bass tab, it means you should put your finger down just before that fret. They count up as you go towards the bass body. A zero (0) indicates an open string. The example above begins with the open D string, followed by an E on the second fret.