The valve lifter/lash adjuster is a hollow cylinder with one end closed off except for a port and a check valve that only lets oil move into the cylinder, The other end has a piston in it, and the piston is pushed towards that end of the cylinder by a spring (a retainer keeps the piston from coming out of the end when the lifter/lash adjuster is not in the engine).
The lifter/lash adjuster on any engine is somewhere in line with the motion of the mechanism that pushes the valve open. As the engine runs, the lifter/lash adjuster is constantly bathed in a drip or flow of oil. If the lifter is empty of oil (full of air), there will be a gap between it and the pieces on either end, so the spring can push the piston outward. That pulls oil into the lifter/adjuster thru the check valve. Then when the gap is closed and pressure is pushed against the lifter/adjuster and the valve, the check valve is pushed closed by air and/or oil inside the lifter/adjuster. The next time the valve is closed and the gap appears, the spring pushes the piston outward again, and more oil is pulled into the lifter/adjuster. When it gets compressed again, air can leak past the piston much quicker than any oil can. This cycle is repeated until all air is out and only oil is in the lifter/adjuster.
Notice that the lifter/adjuster will only be able to fill with enough oil to keep the piston just at the point where there is very little gap, and then can fill no more. So it automatically adjusts the gap to very close to zero. Another word for 'gap' is 'lash'. Hence the name 'lash adjuster'. Also called a 'lifter' because it 'lifts' the valve open.
Below is a sketch from the FSM showing a cam lobe, rocker, and valve stem from the 3.2/3.5 engine. The '2' arrow is pointing to the lifter/lash adjuster. The arrow inside the rocker is indicating flow of oil from inside the rocker pivot rod to flood the lash adjuster.
'98 LXi - Later Concorde gages (black w/ chrome rings)/'99 LX - LHS gages (white) - HIR bulbs