Whole numbers, hundreds and thousands attract limit orders, stop orders and options strikes. Traders set targets at round figures far more often than at arbitrary prices, which concentrates real orders there.
The effect shows up in order book data as clustering, and it is one of the few technical-analysis claims with reasonable academic support: order placement is measurably biased toward round prices. Whether that bias is tradeable after costs is a separate question.
In options-heavy markets the effect is reinforced by strike-price concentration, which can pin price near a round number into expiry. Outside of that mechanism, treat round numbers as places where reactions are slightly more likely, not as reliable levels.
Related: strike-price, support, resistance, order-book