Support and resistance is a concept in technical analysis that the price of a security will tend to stop and reverse at certain predetermined price levels.

Support

A support level is a price level where the price tends to find support as it is going down. 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, even by a small amount, it is likely to continue dropping until it finds another support level.

Resistance                    

A resistance level is the opposite of a support level. It is where the price tends to find resistance as it is going up. 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, even by a small amount, it is likely that it will continue rising until it finds another resistance level.

Support and resistance levels can be identified by trend lines. Some traders believe in using pivot point calculations.

The more often a support/resistance level is "tested" (touched and bounced off of by price), the more significance given to that specific level.

If a price breaks past a support level, that support level often becomes a new resistance level. The opposite is true as well, if price breaks a resistance level, it will often find support at that level in the future.