Congratulations on your pledgeship Paul - you'll have to tell us all a bit more about this group in the UK! Rush is... basically 'recruitment'. The time when fraternities look for new members and potential members look to fraternities. It's a mutual process. They meet up, look around the different groups etc. Sororities do similar - they also have "formal rush" which is a structured process for all the NPC groups (NPC is a governing body for many national sororities - all female fraternities) and that involves rounds of parties, noting preferences down, being offered a bid (place in a group as a new member), bid matching etc!
That's a very broad brushstroke on the whole thing but those are the quick answers
.
Welcome to GC.