Here's the definition from Investopedia.com:
(They're talking about stocks, but it applies to anything that's traded, like commodities)
The price level which, historically, a stock has had difficulty falling below. It is thought of as the level at which a lot of buyers tend to enter the stock. If the price of a stock falls towards a support level it is a test for the stock: the support will either be reconfirmed or wiped out. It will be reconfirmed if a lot of buyers move into the stock, causing it to rise and move away from the support level. It will be wiped out if buyers will not enter the stock and the stock falls below the support.
They have a nice chart that shows an example at:
Support
In a nutshell: it's a price at which buyers tend to come back into the market and "support" the stock (or oil).