The new last modified by field type displays the collaborator who most recently edited the contents of a record. By default, the last modified by field will update when any changes are made by a collaborator in any non-computed fields, but you can adjust the settings in the field customization menu so that the field will only update in response to changes made in specific fields.
A few other notes:
The last modified by cell values will be empty if the last modification predates the time that this feature was shipped (June 2020)
The last modified by field reflects changes in fields where collaborators can make edits. This means if the values change in fields computed by Airtable (e.g. formula, created time, and rollup fields), this will not have any effect on the value in the last modified by field.