@basp sure…ok on a bike you have the large sprocket at the front, your cranks are attached here and its the cog you directly turn…this connects by the chain to a smaller cog at the back wheel. Normally you might have several cogs and a derailleur here on typical geared bikes, the ‘hub’ is the bit the cogs connect to, its what transfers the power from the turning cogs to the wheel itself and you’ll notice on a typical bike you can pedal backwards and the wheel won’t turn backwards at the same time…
On a fixie the rear is ‘fixed’ if you pedal backwards the wheel will spin backwards too…although any bike can technically be fixed hub they’ve recently become pretty cool to own, you can get them custom made for you…there fast as hell but also pretty dangerous, imagine banking into a corner, you HAVE to keep your pedals turning, one clips the ground and bang you come off. But there good at lights with clip-ins because you can rock the bike back and forth at lights etc…also if you ask they can put a different sized cog on either end of the rear hub so you can change to a different gear by popping the wheel out, say for racing or cruising.
If you want one try one out at your local specialist first! Also get a motorbike lock because it will get stolen, they really stand out!