Memberships and credits

Afternoon,

Firstly i must apoloigise for my poor knowledge however i've only ever purchased tickets off here once and that was a straight 'payment up front and tickets in the post'.

So, what's the deal with people looking to sell a membership with (say) 25+ credits? how does that work and what do the credits enable?

I'm looking to sign up as a red member next season and i'm always looking for spare tickets on here however i've never really understood the membership and credits thing.

Any explaination would be handy thanks!