You already know an uptrend makes higher highs and higher lows, and a downtrend makes lower highs and lower lows. Market structure is just the map of those swing points. It is the skeleton under the candles, and once you can see it, a messy chart turns into a clear story of who is in control.
The two points that matter most are the swing high and the swing low. A swing high is a peak with lower candles on both sides. A swing low is a bottom with higher candles on both sides. String them together and you have the sequence: as long as an uptrend keeps carving higher highs and higher lows, the buyers are still winning.
A break of structure is the moment that sequence fails. In an uptrend, the warning comes when price drops below the most recent swing low. That is the first higher low to be taken out, and it says the buyers just lost a floor they had been defending. In a downtrend, the flip is the mirror: price pushes above the most recent swing high.
This is why a break of structure is such a useful signal. It is not a guess about the top or bottom. It is the market itself telling you the old pattern is broken and a new one may be starting. You wait for the level to actually give way, then you trade with the new direction instead of against the old one.
Market structure is the chain of swing highs and swing lows. A break of structure is when that chain fails: an uptrend loses its last higher low, or a downtrend loses its last lower high. That break is your first real sign the trend is turning.
Tip. Mark only the swings that stand out, not every tiny wiggle. If you have to squint to call something a swing low, it is probably noise. The obvious swings are the ones the whole market is watching.