Hydraulic and solid lifters
The lifters are under a lot of stress every time the camshaft presses the valve downwards. A proper lifter ensures that the valve clearance always remains at what it's set to.
Solid lifters are sometimes and overkill for even seriously tuned engines, and the need for them should be carefully considered as they need more adjusting and maintenance than hydraulic ones. Solid lifters also required modifications in some cases, so they are not always a bolt-on solution.