Cancelling tells people. Deleting removes it.
Cancel an event and everyone who said yes, and everyone who has replied anything, is told it is off. Any reminders still scheduled are stopped. The event stays on your list marked cancelled, with the answers intact, so you know who you owe an apology and who you should ask first next time.
Delete an event and it is gone from your list along with its invitations and answers. Links that were sent stop working. Delete is for the one you created twice by accident, not for the one you had to call off — a cancelled event that people were told about is worth keeping.
Neither one charges you, and neither one refunds a paid event by itself. If you paid for an event that is now cancelled, write to help@simpleinvite.app.