As a participant, you can either write a message to everyone at the meeting or to the Host. The default setting is to write to everyone. To change this, click on "Everyone"  above the text field and select "Host".

If you are the Host, you may also choose to write messages directly to individual participants at the meeting. Click on "Everyone" and select a participant from the list of participants.

Everything written to "Everyone" will be visible to every participant at the meeting. 

If you change the recipient from "Everyone" to "Host" (if you are a participant) or to everyone at the meeting (if you are the Host), only the defined recipient will see the message. 

Only those present at the meeting when a message is published in a chat will see the message. Anybody joining the meeting later will not see earlier messages in chats.

Host can change the settings so that it is only possible to write to the Host, only to "Everyone" or not possible to write to anyone: 

Click on the three dots at the top right of the text field and select "Participants can chat with". 

Host may change the settings so that the participants cannot write to anybody: 

Click on the three dots at the top right of the text field and select "Participants can chat with no one". 

Host will still be able to send messages to participants in the meeting. 

Click on the three dots at the top right of the text field and click on "Save Chat".