This may be a stupid question, but I've always heard to never plug a generator into an outlet - what makes these safe enough that you can?

The inverter box contains circuitry that cuts power output within some specified number of milliseconds if unplugged rendering the prongs "cold".

The biggest problem with plugging a generator into an outlet is you need a double ended cord. If you use everything correctly they work fine, but there are too many ways things can go wrong. The most obvious one is you plug everything in, start the generator, and the trip over the cord on you way back inside, thus sending the live male end flying toward you. Once in a while a kid will see these dual male plugs and plug them in - lots of ways things can go wrong here.

There is also the problem of backfeeding, but this is more theoretical - even if you know to limit your power use, your neighbors won't and so your generator will trip its own breaker. Linemen are taught to either treat power lines like they are live, or to short them to ground - it can happen they don't do this and you kill a linemen, but still not as big a worry as it is made out.

They make generator "inlets" that sidestep all these problems when used correctly. Most common I've seen is a block so you can't have the inlet and the main breaker both on at once.

Fancy power electronics in them that make sure they only send power to the outlet while the grid is energizing it too.

In addition to the electronics that others mentioned, they also have limited amperage. That way even if you are pulling from your mains power and the solar at the same time, your circuit hardware shouldn't melt or catch fire.

These sync with the grid and also have safety mechanisms to avoid backfeeding if the grid is down (or it's unplugged).