In short: Support and resistance trendlines are horizontally drawn lines to indicate where the price might end or gain strength.
In stock market technical analysis, support and resistance are certain predetermined levels of the price of a security at which it is thought that the price will tend to stop and reverse. These levels are denoted by multiple touches of price without a breakthrough of the level.
Support versus resistance
A support level is a level where the price tends to find support as it falls. This means that the price is more likely to “bounce” off this level rather than break through it. However, once the price has breached this level, by an amount exceeding some noise, it is likely to continue falling until meeting another support level.
A resistance level is the opposite of a support level. It is where the price tends to find resistance as it rises. Again, this means that the price is more likely to “bounce” off this level rather than break through it. However, once the price has breached this level, by an amount exceeding some noise, it is likely to continue rising until meeting another resistance level.
Support and resistance levels can be identified by trend lines (technical analysis). Some traders believe in using pivot point calculations.
The more often a support/resistance level is “tested” (touched and bounced off by price), the more significance is 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.
Psychological Support and Resistance levels form an important part of a trader’s technical analysis. As price reaches a value ending in 50 (ex. 1.2050) or 00 (ex. 1.3000), people often see these levels as a strong potential for interruption in the current movement. The price may hit the line and reverse, it could hover around the level as Bulls and Bears fought for supremacy, or it may punch straight through. A trader should always exercise caution when approaching 00 levels in general, and 50 levels if it has previously acted as Support or Resistance.
From: Wikipedia “support and resistance”