Usually: A worn CV joint, usually on the outer end of a driveshaft.
But it can also be: A stone in a wheel trim, or a loose wheel trim itself, which is a two-minute fix.
Clicking when you turn the wheel
A clicking sound that only happens when you turn, especially at low speed like pulling into a driveway or turning a tight corner, is usually a worn outer CV joint. That's the joint at the outer end of the driveshaft, near the wheel, and it's what lets the wheel turn while still driving power to it. When the joint wears, the metal parts inside have a little too much play, and you hear it as a click, especially under load.
Before you assume that, though, check the cheap explanation first. A stone caught in a wheel trim, or a wheel trim that's come a bit loose, can make a very similar clicking noise, and it only costs a couple of minutes to rule out. Pull over, walk around the car, and give each wheel trim a firm push and a listen. If one moves or rattles, that may be your whole problem.
How to tell the difference yourself
Find an empty parking lot. Roll the windows down, turn the steering wheel all the way to one lock, and drive slowly in a tight circle. Do the same on the other lock. A CV joint will click rhythmically, in time with the wheel turning, and it'll only do it on full lock, not when driving straight. If the clicking speeds up and slows down with your speed and only shows up on a turn, that's about as clear a sign of a CV joint as you'll get without putting the car on a lift.
If the noise happens on straight roads too, or comes and goes with bumps rather than turns, it's more likely something else, and worth having looked at rather than guessed at.
Is it safe to keep driving
A clicking CV joint by itself is usually safe to drive on for a while, it's not going to strand you the same day. But a joint that's clicking is on its way to failing completely, and a joint that fails completely can mean losing drive to that wheel, or in a bad case, the shaft coming apart while you're driving. So it's not an emergency, but it's not something to leave for months either.
What fixing it involves
If it is the CV joint, the usual fix is replacing the outer joint or the whole driveshaft assembly, depending on the car and what's actually worn. It's a job we'd want to confirm on a lift first, checking for torn boots, grease thrown around the inside of the wheel, and play in the joint by hand, before recommending any parts. If it turns out to be a wheel trim, that's a two-minute job and we'll tell you that instead. Either way, we'd rather look at it and tell you honestly what we find than guess over the phone.