A tax agent is someone who deals with HMRC on your behalf. You're basically authorising them to discuss your affairs with the taxman. Technically, you could authorise someone like a friend or relative to be your official agent. People sometimes do that when they're ill or otherwise unable to handle HMRC themselves. For things like tax refunds and Self Assessment returns, though, people generally turn to the professionals.

Of course, it's possible to muddle through without an agent at all, if you like shuffling your own paperwork. Then again, if you don't know your way around HMRC, you might end up missing out on a refund, finding yourself having to sort out being chosen for an HMRC enquiry or getting tangled up in the Self Assessment system.