Pedal doesn't rotate freely; occasionally comes with grinding / clicking
The pedal bearings are failing in some way. This may range from a ball or race physically broken to a simple lack of grease.

Depending on what kind of pedals you have you may be able to service or replace the bearings. The video linked at the end of this answer shows what's required to do so. It's possible but somewhat of a pain. You have to have the correct tools, if you have to replace balls you have to figure out what size they are etc etc.

If you have simple flat pedals the easiest option is to simple replace them as this type of pedal is inexpensive and easy to get hold of. Pretty much any bike shop should be able to sell you a pair and install them.