The XIRR (Extended Internal Rate of Return) is the internal rate of return of your payments. It converts all payments made at different times into a return per year.
In your account, the XIRR shows the expected net return of all your investments, from the first investment to the end of the last contract. It takes into account interest and late payment interest, repayments, fees and collection.
Because new payments are added all the time, the XIRR changes. It can fluctuate strongly, especially while you are building up your portfolio. The XIRR is a calculation, not a guarantee: if a borrower defaults, it falls.