Day Trading is short term trading.
A trade that is completed within a trading day is day trading. In and out within a day.
Price charts repeat and repeat the same measurement of emotional content in the market.
Throughout the day opportunities exist to pick up patterns in price action that can present better than 50% probability of a directional move in one direction or another.
Double tops and triangles are two of the most familiar types of chart patterns.
Traders look for familiar patterns to take advantage of these opportunities to make money.