- Default: You can create new tasks & rows and modify existing ones.
- Read-Only: You cannot make modifications to any task or row.
If you have read&write permissions then the default mode is active. If you have only read permission (you cannot modify any events in your Google calendar) then the read-only mode is active.
If you would like to change the read-only mode please contact the person which owns the Google calendar. More information on how the owner can change this permission is available in the Google Help Center.