Price does not move in a straight line. It moves in swings: up for a while, then back, then up again. Market structure is simply the pattern those swings make, and every swing begins and ends at a swing high or a swing low.
What counts as a swing point
A swing high is a candle whose high is higher than the highs of the candles on either side of it. A swing low is the mirror image: a candle whose low is lower than the lows on either side.
The key is to choose a rule and apply it every time. A common rule is "N candles on each side", with N between two and five depending on the timeframe. A bigger N finds fewer, more significant swings. A smaller N finds more, noisier ones.
Why consistency matters more than the exact rule
If you mark swings one way on Monday and another way on Friday, you will see different structure on the same chart and every later decision will wobble. Write the rule down. Mark the swings. Do not move them afterwards to fit what price did next.
Internal and external swings
Large swings on a higher timeframe contain smaller swings on a lower one. Traders often call the larger ones external structure and the smaller ones internal. You do not need the vocabulary yet, only the idea that the same chart can be read at more than one scale.