XIRR stands for Extended Internal Rate of Return, and it is the go-to way to measure returns when you've invested money at different points in time — exactly what happens with a SIP, where you invest every month rather than all at once. Ordinary return calculations assume a single lumpsum invested on one date, which doesn't fit a SIP where each instalment has its own start date and has been growing for a different length of time.
XIRR accounts for the exact date and amount of every single instalment, along with the final value, to calculate one accurate annualised return figure. This makes it far more reliable than simply averaging monthly returns when you're checking how your SIP has actually performed. Most mutual fund apps and portfolio trackers calculate XIRR automatically, so you rarely need to compute it by hand — just know that it's the right number to look at for SIP performance.