The crankset is the chainrings and the crankarms. The bottom bracket is the spindle, left and cups and the ball bearings.

If it grinds when rotated then the bottom bracket needs servicing or replacement. You need to remove the crankarms and the lef side adjustable cup to be able to service the bottom bracket. To do this, you need a crankarm puller, and a couple of wrenches to remove the adjustable cup.

If either cup (fixed or adjustable) or the spindle is/are damaged it's more economical to replace the bottom bracket with a shimano UN54 or UN55 cartridge bottom bracket ($20-$25 online) then it would be to replace the damaged cup/s and or spindle. Measure the length of the spindle to figure out what length cartridge bottom bracket you need. The length of the cartridge you need is usually the same as the length of the existing spindle.