Large addresses are visible to anyone, and services publish alerts when they move. The trouble is attribution: the biggest addresses are usually exchange or custodian cold wallets holding other people's coins, so a 10,000-coin transfer is often an internal rotation.
Even correctly identified, intent is unknowable. A transfer to an exchange may fund a sale, post derivatives collateral, or move to a different custodian. Sophisticated holders also split across many addresses precisely to avoid the attention that alerts create.
Whale-watching is worth most as context for thin markets. If a handful of addresses hold most of a small token's float, that is a concentration risk you can verify on a block-explorer before buying, and a much more actionable fact than any single transfer alert. See whale.
Related: whale, exchange-netflow, block-explorer, exit-liquidity