What it means
In an uptrend it is a close below the last higher low. Traders usually wait for a lower high and a break of structure down to confirm.
In practice
A change of character is the first evidence that the side in control has lost a round. In an uptrend, sellers manage to close price below a higher low that buyers had been defending.
Traders treat it as a reason to stop looking for trades with the old trend and to start watching for the new one to prove itself: a lower high, then a break of structure down.
What to watch for
One close through one level is thin evidence. Many are just deeper pullbacks, especially when they occur against the higher-timeframe trend, and price resumes the original direction soon after.