Usually: A wheel bearing, which typically changes as you steer gently left and right.
But it can also be: Tire wear — cupped or feathered tread hums in a very similar way and is far cheaper to fix.
Humming that shows up at highway speed
A steady hum or drone that starts somewhere around 40-45 mph and gets louder the faster you go is usually a wheel bearing. The bearing lets the wheel spin freely, and as it wears, it starts to complain under load. The other common cause is uneven tire wear — cupping or feathering on the tread — which moves air in a way that sounds a lot like a bearing to most ears. Tire wear is the cheaper of the two by a wide margin, so it is worth ruling out first rather than assuming the worse case.
A test you can do yourself
Find an empty road or a stretch with no traffic behind you. At a steady speed, gently weave the car left and right — just enough to shift the car's weight from one side to the other, nothing sharp. A worn wheel bearing usually changes its note as you do this, because you're loading and unloading the bad bearing with each shift. Tire noise generally does not change with this kind of weaving — it stays about the same regardless of which side is loaded. It is not a perfect test, but it is the single best thing you can check in a parking lot before bringing it in.
Is it safe to keep driving
In most cases, yes, for a while. A humming bearing that hasn't gotten worse (louder, or joined by a grinding or clicking sound) is usually fine to drive on for days or a couple weeks while you get it looked at. That said, a bearing that fails completely can let a wheel shift or seize, which is a real safety issue, so this is not something to let go indefinitely. If the noise gets noticeably worse, or you feel any looseness or vibration in the steering, treat it as more urgent.
What fixing it involves
If it's a wheel bearing, the fix is replacing the bearing (and sometimes the hub assembly it's built into) on the affected side. It's a specific job, not usually something a driver would tackle at home. If it turns out to be the tires, the usual fix is rotation, an alignment check, or replacement if the wear is too far along — and if it's just uneven wear from an old alignment issue, that's worth fixing too, so it doesn't just wear out the next set of tires the same way. Cost varies quite a bit depending on which one it is and which wheel, so it's best to get it checked and diagnosed properly rather than guess and buy a part that isn't the problem.