Camshaft bearings enable the camshaft to move around freely. You will find them between the camshaft and the engine block. They should always be replaced when you overhaul an engine if your rod or main bearings fail. Whatever dirt or debris that damaged those parts most likely have damaged the camshaft bearings as well.
If you find metal flakes or shavings in your engine oil next time you do an oil change, this is a pretty good sign that the camshaft bearings are ready to be replaced. If they are worn you might also notice low oil pressure when the engine warms up and you might also hear chirping noises. The chirping noise coming from the camshaft bearings can be caused by a lack of lubrication or just everyday wear and tear.
Camshaft bearings that are worn should be replaced as soon as possible to prevent a complete breakdown of the engine.