A break of structure (BOS) is a candle close beyond a prior swing high or low in the direction of the prevailing trend: above the last swing high in an uptrend, or below the last swing low in a downtrend. It is read as confirmation that the trend is continuing: the market has printed a new higher high (or lower low).
BOS is the continuation counterpart to the market structure shift (MSS), which is a break against the trend. Together they form the grammar of structure-based trading: BOS events keep you with the move; an MSS warns the move may be over. Mechanical implementations confirm swings with a fractal lookback and require a close, not just a wick, through the level.
Usage varies between communities (some use BOS for any structural break regardless of direction and reserve CHoCH for reversals), so when reading or testing a strategy, pin down which convention it follows first.