To prevent dust accounts from bloating the chain state, Polkadot stipulates that if the balance of an account is less than 1 DOT, the remaining DOT will be burned. To avoid this, please make sure that the remaining DOT in your account is always greater than 1.

Further reading: Polkadot Protocol Information

Note: If your address is using DOT network's staking service, you don't need to worry about this issue.