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 has 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.