With gmail if you have an account like example@gmail.com you can then sign up for a website such as netflix with email example+netflix@gmail.com and gmail will forward it to example@gmail.com, but you’ll still see the full address on the To line so you’ll know where the mail came from. Anything after the + can be whatever you want. This lets you sign up with a different email address for every site you visit without having to create new addresses with gmail. You can also make a filter to hide spam if one of the addresses is compromised.