What do the numbers mean on the guys' backs?

Each player in a rugby team has a particular match-play role within the team and hence a corresponding position/jersey number.

There are 15 players in the team, eight 'forwards' numbered 1 - 8 and seven 'backs' numbered 9 - 15:

1 Loosehead prop; 2 Hooker; 3 Tighthead prop (the front row)
4 Lock; 5 Lock (the second row)
6 Blindside flanker; 8 'No 8'; 7 Openside flanker (the back row; flankers are also known as breakaways)

9 Scrum-half/half-back
10 Fly-half/five-eighth
11 Left wing; 14 Right wing
12 Inside centre
13 Outside centre
15 Full-back

You may hear guys or the TV commentators refer to 'the pack', 'the midfield', 'the back three' etc. These are collective terms for the forwards (all 8 of whom make up the scrum), the centres (players 12 & 13), and the wingers & full-back.

Generally you'll see players numbered 1 - 8 packing down into scrums and lining up into 'line-outs' to catch the ball, and players 9 - 15 throwing the ball around between them and running with it to the try line.

Forwards are generally big and strong physically, whilst the backs are fast runners.

I usually refer to the forwards as 'the big fat forwards'. Look at a few of them and you'll see what I mean. Big fat necks to support them in the scrum.

I refer to the backs as 'the fancy boys'. They are often better looking than the forwards. The famous Australian back David Campese was famous for not tackling anybody because he liked to keep his jersey clean. This why they're fancy :-)