Market structure is a way of describing price action through the sequence of swing highs and swing lows it forms. A series of higher highs and higher lows is commonly described as an uptrend structure, while lower highs and lower lows describe a downtrend. When highs and lows stay within a band, the market is said to be ranging.
Traders and analysts use structure to frame context: identifying when a prevailing pattern is intact and when a key level has been broken, which is often called a change or break of structure.
Market structure is a descriptive framework, not a predictive rule. It organizes what price has already done so that other observations can be placed in context, and different observers may read the same chart differently.