As others have mentioned, check the hub/axle/bearings.

Remove wheel, and spin the axle. It should turn effortlessly, smoothly, and with any grabbing.

If the bike is old, and hasn't been "tuned" for some time, you're probably due for a full service, including taking apart and cleaning, lubing, and adjusting all the cones and bearings.

It is something that anybody can do, but it does require a few specialized tools, and is a little fiddly.

For the rear hub, you'll need a freewheel wrench that matches your freewheel, and a 14 or 15mm cone wrench, and a 17mm (probably) standard wrench. Plus some bicycle grease and a rag. The front wheel will take a 13 or 14 mm cone wrench. The bottom bracket and headset should also be serviced sometime.

If it has been abused, maybe also new cones and bearings. Perhaps a new axle. And, if really bad, a new hub or wheel.