@Emma239 - Last year when the guests checked in, 24 hours later you were paid in full for the entire reservation. When you agreed to a refund, that needs to come back out of funds paid to you. Since it sounds like you've had no other guests since then, it is being deducted now. If the guests had a medical emergency and were able to provide proof to Airbnb that the had "Extenuating Circumstances" they are entitled to a full refund and those funds get paid from your earnings.
You can read about these policies:
https://www.airbnb.com/help/article/1320/what-is-airbnb-s-extenuating-circumstances-policy
https://www.airbnb.com/terms/payments_terms
"If, as a Host, your Guest cancels a confirmed Booking or Airbnb decides that it is necessary to cancel a confirmed Booking, and Airbnb issues a refund to the Guest in accordance with the Guest Refund Policy, Extenuating Circumstances Policy or other applicable cancellation policies, you agree that in the event you have already been paid, Airbnb Payments will be entitled to recover the amount of any such refund from you, including by subtracting such refund amount out from any future Accommodation Fees due to you."