Volume is a very useful indicator. It represents how much stock as been traded in one trading day. If the amount is 200,000, then that is how many shares of that particular stock has been traded (or changed hands, if you prefer) that day. A rule of thumb is that a stock with high volume is easier to trade. More Traders are interested in it, that's why the volume is high. Low interest, low volume.
It's a lot easier to sell your stock if it enjoys a high amount of volume, when it achieves the price that has been anticipated. This is because there are a lot more Traders out there waiting to buy it, than say with one that only churns up about 20,000 or even less a day.