The default calendar is the calendar that is selected automatically when creating an event.
For example, if you mostly create events for work, setting your work calendar as the default means you rarely need to change the calendar manually.
It is worth noting that you can change the calendar to any other linked calendar when making an event. The default is just there to save time.
To change your default calendar, navigate to settings by clicking your name/email in the bottom left and clicking the “Settings” tab.
Then, select “Calendar”. Here, you will see a “Default Calendar” section. Clicking the drop-down here will show all of the linked calendars you have. Select your most used calendar, and it will become the default.