@Pauline865 It’s the fee Airbnb charges the guest for every reservation. For the guest, the fees usually run around 12% of the rent and cleaning fees, although I don’t think any of us know the exact figure - it seems to vary somewhat. You as the host pay Airbnb fees, too.
The fees are Airbnb’s source of revenue, and go towards marketing, running the website, collecting and processing payments, customer support, software development, and a myriad of other things. The fees are the reason that you have a well-known, easy to research and navigate web portal where your guests can find and book your accommodations.