L2=Market Depth. It tells you the size of each bid or ask amount, and the number of buyers or sellers at each price point.

In my case, my brokerage tells me the number of lots on offer at each price, not the number of shares. So for Century, it tells you number of 500-lots; for a stock sellling over $1, it'll give you number of lots of 100. For a penny stock, lots will be of 1000.

Basically, to determine the day's reasonable expectation of support, look at how many bids are there at which prices. If someone's got a standing bid to buy 500,000 shares at 43 cents, you can be pretty sure the stock price won't drop below there without a high volume of selling into the bids.

If the stock's trading at 48 cents today, but you see very few bids til you hit 44 cents, then it's possible the stock will drop because there's insufficient buy support.

Similarly, if it's at 45 cents today and you see 500,000 shares being offered to sell at .46, you can be pretty sure that the stock won't go up past that, unless volume increases greatly. And if its price is .45 today and there are very few asks until .50, you've got a good chance of the stock moving upwards that day if the demand is there.

The big houses and rich-people's brokerages can post invisible buys and sells, though, so you never really know what's available to any great degree. Plus, with guys like me, I don't put a bid in and let it sit there for people to see; I wait til the stock hits 43 cents and then put in a quick buy order for that day. Then if the stock hits .52, I look to see how much bid support is there, and then usually just dump into the market with a market sell order. I've got enough profit anyway, and I don't want to put in a limit sell at .52 because then a bunch of other people who can see the L2 will butt in front of me and sell at .51, then I'll never manage to sell anything.

I only get the L2 because I trade frequently and have more than a certain number of $$$ with my discount brokerage. Other people here don't have access to L2 data, and as you can see it's very useful to people, especially when they're in a rush to buy or sell but want to get the best price. So, people like us with access to L2 like to post the data for everyone to see - especially on "critical" days for a stock - so more people can make more money that day.