A swing high is a local turning point at the top of a price move. The strict definition: a candle whose high is higher than the highs of the candles immediately to its left and right (the simplest version uses one candle on each side; longer-window definitions use two or three). Practically, swing highs are the peaks that catch the eye when you look at any chart — the points where price stopped going up and started going down.
Swing highs are the structural input for almost everything: they define trend (higher swing highs = uptrend, lower swing highs = downtrend), they mark candidate resistance zones, they anchor trend lines, and they form the invalidation references for short trades (a close back above the most recent swing high typically invalidates a bearish bias).
The size of the swing high matters. A small intraday peak on a minute chart is a swing high but a noisy one. A peak that catches the eye on the daily or weekly chart is a significant swing high and carries more structural weight. Always identify swing highs on the time frame relevant to your trade — higher-time-frame swing highs are more meaningful than lower-time-frame ones.
The earliest structural warning an uptrend may be ending is a lower swing high — an attempt to extend that falls short of the prior peak. This precedes the lower-low confirmation by at least one swing, which is why traders watching structure can position earlier than traders watching only indicators or moving averages.
Swing highs are also where stops cluster. Many traders place stop-losses just above the most recent swing high on a short trade — meaning a sweep through that swing high can trigger a wave of buying that briefly extends the move before reversing. Identifying where the obvious stops sit is part of why marking swing highs is useful even on trades you have no intention of taking.