Occasionally, attackers will send tiny fractions of bitcoin (around 500 sats) to random wallets. If the owner of the wallet does not notice, they may inadvertently include this piece of dust in their next transaction, leaking information about which bitcoin they own to the attacker.
A dust attack is like placing a tracking device on a victim, allowing the attacker to view where the victim travels, or in this case, which bitcoin the victim owns. High quality wallets do their best to disallow users from including dust in their transactions in order to protect them from such attacks.
If you notice tiny amounts of unsolicited bitcoin being sent to your wallet, you should not spend them as part of a transaction with other pieces of bitcoin. A dust attack only works if the victim spends the dust along with other UTXOs they own.