A folio number is a unique account identifier that a mutual fund company (called an AMC, or Asset Management Company) assigns to you when you first invest with them, used to track and consolidate all your investments, transactions, and holdings with that particular fund house under one reference number.
If you invest in multiple different schemes offered by the same fund house, you can often choose to hold them all under a single folio number, making it easier to view and manage everything together, rather than juggling separate folio numbers for each individual fund. Your folio number, along with your PAN and KYC details, is what you'll typically need to provide whenever you want to check your holdings, make additional investments, or redeem units with that specific fund house.
It's a good idea to keep your folio numbers noted somewhere safe, alongside your other investment records, since your family may need them to trace and claim your investments in your absence.