Suffix "+1" in email addresses?

Hello, I regularly receive emails addressed to me with a modified version of my address. This address has the suffix +1 before the at symbol.

For example, let’s say my address is ***@***, I find myself receiving emails sent to ***@***

What does this +1 mean? What is its purpose? How is it that the mail still reaches me even though it doesn't correspond to my address?

I can't find any answers on the internet. Even the technical support team at Outlook can't seem to answer me...

Thank you very much in advance for your responses.