Trading volume measures how much of an asset changed hands over a given period, such as a day. High volume means a lot of buying and selling activity, while low volume means relatively little. It is one of the most common figures shown alongside price.
Volume is useful because it gives context to price movements. A price change on high volume reflects broad participation, while the same move on very low volume involves fewer participants. Volume also relates to liquidity, since more active markets are generally easier to trade in.
For beginners, volume is simply a gauge of activity. It does not predict direction on its own, but it helps you understand how much interest there is in an asset at a given time, which adds context to what the price is doing.