Tablature or TAB is a really great way to communicate finger instructions on the guitar, and its simple enough that a beginner can start reading it with very little frustration. There are enough challenges with playing the guitar already without figuring out notation straight away! TAB does have some pretty serious failings and it is important to get into standard notation as soon as you can.
TAB has six horizontal lines, each one represents a string on the guitar. The top line represents string #1 E and the bottom is #6 low E. When there is a number on that line, it means to play the note at the corresponding fret number. For example, if there is a 5 on the top line, it means play the fifth fret on the first string. If there are numbers stacked on top of each other over different strings it means that you have to hold down all those notes and play a chord.
And that is pretty much it! Once you get into electric guitar TABs they can get pretty complex and they have a whole set of symbols that you can learn for bends, shifts etc. For classical guitar, it is more of a pre-cursor to reading notation. I mentioned earlier, that TAB had some drawbacks, some examples are a lack of rhythm, it doesn’t tell you what note you are playing and therefore harmonies can be a mystery, and also it only tells you one way to play a passage. One of the beauties of the classical guitar is that you can play notes in several different places depending on your musical choices, TAB just tells you one way to play the passage.