XC stands for Cross Country. Rider's go round and round a loop course. This is Round 2 of the club's 5 round series but don't worry if you missed round 1, that's OK, anyone can join in at any round. The number of laps you do depends on the category you want to enter (see Categories on the event promotor website).
Not sure but still want to have a go? Enter the Novice event; this is a shorter course but over a fixed time - you can do as many of these shorter laps as you like (or can). It'll be a load of fun.
The course is about 7.8km; there'll also be a shorter Novice course (approx 2km)