Usually: Uneven tire wear, often from an alignment problem that has been there a while.
But it can also be: A wheel bearing, or on some cars a differential.
Droning at highway speed
A steady drone that shows up once you get up to highway speed, and changes pitch with how fast you're going, is usually tire wear. Not a defect in the tire itself, most of the time, but uneven wear across the tread — cupping, feathering, one edge worn more than the other. That kind of wear is often a sign the alignment has been off for a while. The tires wear that way, and they hum because of it.
Here's a way to narrow it down without any tools. Find a quiet road and notice whether the sound changes when the road surface changes — worse on coarse chip-seal, quieter on smooth pavement. That points to tires. Then, if you can do it safely, take your hands loose on the wheel for a second on a straight, flat road and see if the car pulls to one side. A pull, along with the drone, points more toward alignment as the underlying cause. Also try this: does the pitch change if you gently weave left and right at low speed in an empty lot? A wheel bearing will often growl or change pitch with the load shifting side to side, more than with road surface. A differential noise, on cars that have one, tends to show up more as a whine or hum tied to engine load — worse accelerating, quieter coasting — and on those cars only.
None of these tests are perfect on their own. That's the honest answer. But tires and alignment together are by far the most common cause of this specific complaint, so that's where we'd start.
Is it safe to keep driving? Generally yes, in the short term. A drone by itself is not usually a sign of something about to fail. That said, a bad wheel bearing can get worse and, in rare cases, fail outright, so if the sound is getting louder over days or weeks rather than staying steady, don't wait too long.
Putting it right usually starts with a look at the tires and a road test to hear the noise ourselves. If the tires are uneven, we check the alignment, because replacing tires without fixing the alignment just wears the new ones the same way. If the tires and alignment check out fine, we'd check the wheel bearings next, and the differential if the car has one. Cost varies a lot depending on which of these it turns out to be, so we won't guess a number until we've actually heard it and had a look.