To start, Wabis can run single speed (i.e. freewheel) or fixed. Every build we ship actually comes with a freewheel or fixed gear on the flip-flop hub so you can ride both ways by flipping the rear wheel.
Now, let's discuss the difference especially as some people will sometimes use these terms interchangeably (though they are different).
Both single speeds and fixed gear have single gears (or gear ratios) that use one size of chain ring at the crank and one size of cog or freewheel at the rear wheel. This means you only have one speed (or gear) when riding.
Single speed is when the rear hub uses a 'freewheel' that allows the rear wheel to disengage from the chain (and crankset) when the rider stops pedaling. This is how most bikes work and so single speed is similar to riding any bicycle - road bikes, mountain bikes, cruisers, etc. You just don't have other gears to shift into like traditional road bikes or mountain bikes.
Fixed gear (aka 'fixie") is when the rear hub uses a fixed cog that always engages the rear wheel with the chain (and crankset). This means the pedals never stop moving even if you stop pedaling. This is definitely a different way of riding for most people and takes some practice. However, once you get used to it, riding fixed gear offers a new way of experiencing riding.
With Wabi, you can use either or both. Our flip-flop hubs allow you to have a single speed 'freewheel' on one side and a fixed cog on the other. You just have to loosen the wheel bolts, slide out the wheel, flip it over and retighten the wheel bolts. This is great for those that want to ride both ways or may want to start with single speed before they move over to fixed gear.