Usually: A wheel bearing, which usually also hums at speed.
But it can also be: A brake component contacting the disc only when the suspension loads up in a corner.
Grinding when you turn the wheel
This one is worth stopping for, not driving on for another week to see if it gets worse. Grinding is a step past the humming or droning noise a wheel bearing usually makes first, and a bearing that has already moved from humming to grinding is further along than it sounds.
What it usually is
Most of the time this is a wheel bearing. The classic pattern is a hum or drone that changes with road speed, then gets louder or turns into a grind when you turn the wheel one direction, because turning shifts the load on the bearing and makes play in it more obvious. If your car has been humming at speed for a while and now grinds on turns too, that fits a bearing that has worn past its early stage.
It is not the only explanation, though. A brake component — a caliper bracket, a worn pad tab, a piece of hardware — can also contact the rotor only when the suspension loads up in a corner, which produces a similar grind that shows up only when turning. This one tends not to have that background hum at speed.
How to tell them apart
A rough way to sort the two, in a parking lot or on a quiet street: get the car moving slowly and turn tightly one way, then the other. If the noise is worse turning one direction than the other, and there's also been a hum or drone on the highway that changes with speed, bearing is the more likely answer. If the noise only ever shows up in a corner and there's no highway noise at all, look harder at brake hardware first. Neither test is definite — this is a case where having someone put the car on a lift and turn the wheel by hand, watching and listening directly, settles it a lot faster than guessing from the driver's seat.
Is it safe to keep driving
Drive it gently and get it looked at soon rather than putting it off. A wheel bearing that has already gone from humming to grinding can fail completely, and a bearing failure at speed is not a good place to find out you needed the appointment last week. Short trips at low speed to get it in are reasonable. A road trip is not.
What the fix generally involves
If it's the bearing, that means replacing the wheel bearing and hub assembly on that corner — not something with a quick workaround. If it's brake hardware contacting the rotor, it's often a smaller job, sometimes just hardware and a pad replacement. Either way it needs an on-the-lift look before anyone can tell you which one you've got, and roughly what it will run.