In an uptrend, a change of character is the first close below a prior swing-low after a run of higher lows. It says the pattern that had been holding has failed at least once.
It is deliberately an early signal and therefore an unreliable one. Many changes of character are just deep pullbacks inside a trend that then resumes. Used alone the false rate is high; used as a reason to tighten stops, stop adding, or wait for more evidence, it is sensible.
The usual sequence traders look for is a liquidity-sweep that takes out stops, then a change of character, then a retest of the level that broke. Nothing about that sequence guarantees anything, but it at least demands several independent things to line up before committing.
Related: break-of-structure, market-structure, liquidity-sweep, swing-low, trendline-break