XIRR is that single rate of return, which when applied to every instalment (and redemptions if any) would give the current value of the total investment. XIRR is your personal rate of return. It is your actual return on investments.
XIRR stands for Extended Internal Rate of Return and is a method used to calculate returns on investments where there are multiple transactions happening at different times.
In the case of SIP, since there are multiple investments (therefore multiple purchase prices) and different time periods for each instalment, calculating returns is a bit tricky. Returns on Mutual Fund SIP are commonly done in terms of XIRR.