Hey @TheGOoDSweets and welcome to the Community.
When you Void an item, you remove the item from the bill entirely. This is used often in restaurants when a mistake is realized after an order is already prepared.
When you Comp an item, you remove the cost of the item, but the item itself is still on the bill and usually still given to the customer e.g. you make an extra dessert by mistake, but still bring it to the customer as a goodwill gesture or a drink took longer than expected to arrive so you comp the drink to make up for the wait time.
This Support Center article goes into detail about the Comp & Void feature and how to use it within your Point of Sale app:
Hopefully this helps, and let me know if you have any other questions.