Price rarely moves in a straight line. It keeps stopping and turning at the same levels. A level where price keeps stopping on the way down is support. A level where it keeps stopping on the way up is resistance.
These levels matter because they are memory. Buyers who stepped in at a price before tend to step in there again. Sellers who took profit at a price before tend to sell there again. The more times a level holds, the more traders watch it.
One more thing to know: when support finally breaks, it often becomes resistance, and when resistance breaks, it often becomes support. The floor becomes the ceiling, and the ceiling becomes the floor.
Support is a floor where buyers keep stepping in. Resistance is a ceiling where sellers keep stepping in. Broken support often flips to resistance, and the reverse.
Tip. Draw levels where price actually reacted more than once. A level is only worth watching if the market has already respected it.