Support & resistance
Support is a price where buyers repeatedly step in and stop a fall. Resistance is where sellers stop a rise. These levels form because the market remembers them.
How to use them: look for price to bounce off a level (trade the reaction) or break through it (trade the breakout, ideally on a retest). The more times a level is touched, the more significant it becomes — until it breaks.
Round numbers (1.1000, 1900.00 on gold) often act as psychological S/R.