hydraulic lifters - question

Hi guys - excuse my ignorance with this question -

My engine makes a tapping noise, which has been diagnosed as "hydraulic lifters are stuffed" - one mechanic told me I had to get this fixed as soon as possible or experience serious engine trouble. Another mechanic told me it wasn't a serious problem and not to get it fixed yet.

My question is - what are hydraulic lifters, what do they do?

And if they are stuffed, how urgent is it that I get them fixed.

My vehicle is a Falcon ED 1993, 4.0L, auto, s/w

Thanks

I have never heard of "stuffed" lifters ... he/they might mean "stuck".

A lifter is an adjustable mechanical device that sits on the top of the camshaft. As the lobe of the camshaft rotates it lifts the lifter and pushrod. the pushrod rocks the rocker(it is like a little teeter - totter with the valve on one side and the puchrod on the other) which depresses the valve. The hydraulic lifter take up all the clearances between the cam and lifter,pushrod,rocker,valve for a quite opperation.
Listen to an air cooled VW next time... all that clacking you hear is comming from the valves and lifters.