The crankshaft is an engine component that rotates as pistons (connected to the crankshaft via piston rods) use combustion energy to push downward on it at the appropriate time for each piston.
The crankshaft has a section for each piston rod, and each of those tend to have a set of counterweights which keep it balanced, which reduces engine vibration. Some naturally balanced piston engine layouts don't need much counterweight to run smoothly.
The ends of the crankshaft are held by the main bearings, which are sealed off by the front and rear main seals. There are other support bearings found inside of the engine block, which don't need to be sealed off because both sides are inside the block.
The piston rods have rod bearings within their bottom bore, which rides on the rod journals machined into the crankshaft. The journal has to be precisely machined and the rod bearings need to be sized to match in order to have a precise gap for oil to flow into. This gap creates a layer of oil that, under ideal conditions, acts as a fluid bearing.
Rod journals start out with a polished appearance and ideally remain that way, but if problems arise, they may become scored. Rod bearings are designed to be the wear item in this area, but aren't always the only item that gets worn.