You can exchange private messages with other users on a one-to-one basis. The messages are only visible to the sender and recipient.
This is beneficial when you want to contact a specific user privately. For example, you can use private messages to ask your supervisor or senior colleague for advice, or to have a casual talk with your co-worker.

To send a private message to a user, search for the user from the search box at the upper right of the screen, then click the Send private message icon to the right of the user name.

When the Messages screen opens, enter your message and click Send.

You can also send your message from a user's People page.
Search for the user name to open the user's People page, or click the Quick Access Menu icon at the top of the screen and click People.
The list of Kintone users appears.

Clicking on a user opens the user's People page.
When you click the Send private message icon on the right side of the People page, the "Messages" screen opens.