A USB caddy is an external enclosure that can have the hard drive from your laptop fitted into it, you then plug the enclosure into another computer and you can access all your files that were on the drive.
To be honest, if your computer is in a repair shop I'd be tempted to ask them to supply a USB caddy and fit the drive into it for you (unless they try and charge much more than about £25 for the caddy and about £10-15 for fitting the drive)
If you want to do it yourself and save a bit of money you can buy caddies for about £15 (+p&p) off the internet but you need to make sure you get one with the correct type connector in it for your hard drive.