Tab is short for tablature. It's a different way of reading music than you may be used to: instead of each bar representing a certain note, they each represent a string. They are arranged so that the high E is represented by the top line and low E is represented by the lowest bar. If the strings use an unconventional tuning, the tuning will usually be shown next to the lines so you can see how each line should be tuned.
Unlike conventional written music, time values are very rarely shown in the tab. Instead, an estimate of how long it is between every plucked note is commonly shown to roughly estimate timing.