In addition to Ganesh Sanap’s Answer:
This is a new feature named Microsoft Lists: comments – built on SharePoint, currently rolling out in SharePoint Online.
The feature is not possible to be disabled at the site or list level currently, but only at the organization level by changing the
CommentsOnListItemsDisabled parameter in the
Set-SPOTenant PowerShell cmdlet.
You must be a SharePoint Admin or Global Admin in your tenant to turn the feature off.