The 52-week high and low are the highest and lowest prices a stock has traded at over the trailing twelve months. They're a quick way to see where the current price sits within its own recent range — near the top, near the bottom, or somewhere in the middle.
Some traders watch the 52-week high specifically as a technical level: a stock breaking through its own 52-week high is trading somewhere it hasn't been in a year, with no recent sellers at that price to absorb further buying — a pattern some momentum strategies are built around. The 52-week low gets the opposite kind of attention, sometimes read as a value signal and sometimes as a warning, depending on why the stock got there.
The range resets continuously — it is always the trailing year from today, not a fixed calendar year — so it quietly drops old highs and lows off the back end every day.