Hydraulic valves are valves where the clearance is controlled by means of hydraulic oil pressure. This is in contrast to adjustable valves. These must be adjusted manually. The valve lifter is located and moves between the valves and the camshaft. In the hydraulic variant, the clearance is therefore controlled by means of oil pressure. The valve lifters are filled with oil via an oil supply channel. The moment pollution is present here, the clearance of the valve lifters will no longer be optimal, so that the engine can make some noise. This is often where the ticking sound in an engine comes from.
Hydraulic valves therefore have the advantage that they do not have to be adjusted manually and usually they also work more quietly than adjustable valves. The big disadvantage is that they can easily cause problems due to internal pollution.