Support and resistance identify areas of supply and demand. But what exactly is supply and demand?

Supply is an area on a chart where sellers are likely going to overwhelm buyers causing the stock to go down. On a chart, we call this resistance.

Demand is an area on a chart where buyers are likely going to overwhelm sellers causing the stock to go up. On a chart, we call this support.

Knowing this, it only makes sense to buy at support and sell at resistance!

Stocks run into resistance (supply) because those traders that bought too late and saw the price go down now want to get out at break even so they sell. Stocks find support (demand) because those traders that missed the move up now have a second chance to get in so they buy.