Support level is the price level, which, historically, a security has had difficulty falling below. Typically, the support level can be thought of as the level at which many buyers tend to enter a security.  Support levels are often used in technical analysis.
The original low price level becomes a support area because investors will believe that prices will again rise above that level.  This means the price is more likely to "bounce" off this level rather than break through it. However, once the price has passed this level, by an amount exceeding some noise, it is likely to continue dropping until it finds another support level. 
From 14c., from, bring up, up from under, portare "to carry." Sense of "that which supports" is from 1560s.