L2 means "level 2 quotes". It shows the number of shares each market maker has bid, and at what price, and also the number of shares each market maker has for sale and at what price.
More precisely, it shows you the highest bid and lowest ask for each MM, but not the depth of orders behind that at each MM.
Your normal "stock quote" is "level 1" - you see only the high bid and the low ask. You don't see how many shares, from what MM, whether there are multiple MM's at the inside bid or ask, or whether the "inside" bid or ask is something of an outlier (i.e. an inside bid under which there is a decent size price gap down to the next highest bid) or is backed by a bunch more orders close to that number.
Level 2 answers all of those questions. Looking at level2 can give you strong clues regarding the prices at which orders of varying sizes might fill.
Hope that helps. You could probably get a fuller answer if you were to Google something like "level 2 stock quotes"
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