You can attach encrypted or public comments to transactions. They will be stored in the Free TON blockchain.
All can see your encrypted comments in the blockchain as an encrypted string and the content is not available to third parties. For you and your counterparty it will be a normal text in Surf.
All unencrypted comments to transactions are public in the Free TON blockchain. It is possible to check it through ton.live. Your comments, in particular, can be founded by the following actions: click on a transaction with comment in the ton.live → Out messages ID → More details → Body UTF-8.
Please take into account that the comment will increase your fees. The comment is a body of the transaction and increases the size of the transaction, that make and influence during the Free TON fee calculation.