How It Works
Support, along with its cousin, resistance, are extremely important concepts in swing trading, and they are predicted by drawing horizontal trendlines on a stock price chart. These trendlines identify trading opportunities.
At least two highs or two lows in the same area are required to create a support level. The pattern gains greater validity every time the stock "tests" price support levels by bouncing off of it.
Let's look at a chart for the hypothetical stock TREN:

The chart shows the following:
> R1-S2. TREN trends sharply downward. It finds support in early August at $35, where it then moves sideways, or "consolidates," for several days.
> S2-R2. The shares rally briefly from roughly $35 to $40. Notice how we can connect R2 with S1 by using a straight line. What had been for a short time a support level has now turned into a resistance level.
> R2-S3. Between mid-August and early September the shares drop precipitously, from $40 to $30.
> S3-R3. The rally is brief. TREN recovers to $35. What had been support at S2, $35, has now turned into resistance at R3.
> R3-S4. A mid-September decline takes TREN from $35 to $27.
> S4-R4. TREN pushes back to $30, the level at which it had previously found support (at S3).
> R4-S5. The shares decline to their final early October low of just over $25.