Shower drain leak?

...noticed a leak this morning seeping into the carpet and it appears the drain area is no longer snug and seating securely in the tray, like it should. I've removed the front panel and it seems dry everywhere else around the tray.

Is it a case of removing the drain hole fitting and replacing? If so, what keeps drain holes fitting tightly and waterproof in the first place?

Thanks

rubber washers / occasionally silicone

Thanks Kevplumb, the washer it is then. Cheers.