Can someone explain the Time Machine to me on a Mac....

[domesticallychallenged]

....I know it's for backing everything up and I seriously need to do this but do I need to buy an external hard drive or something? TIA

yeah - you need an external drive to back up to. If you back up to your Mac and the worst happens you lose your hard drive and the back-up - worse than useless. Apple's Time Capsule is designed to integrate with Time Machine seamlessly and will do the backing up wirelessly (after the first back up which it needs to be wired for), but you can use any external drive. Firewire drives tend to be quicker than USB