Dust is an amount too small to be worth spending. An attacker sprays it across addresses, then watches for the moment a wallet includes that dust as an input alongside other coins, which links the addresses to one owner.
The goal is deanonymisation for profiling, extortion or targeted phishing. On utxo chains the linkage is structural, which is why privacy-focused wallets let you freeze specific outputs and exclude them from spending.
Practical response: do not spend unexplained dust, mark it as do-not-use if your wallet supports coin control, and ignore any accompanying token or message, which is usually bait for a wallet-drainer site.
Related: utxo, address-poisoning, wallet-drainer, crypto-address