When they are talking about gap or lash and pre-load, they are referring to the space between the top of the valve stem and the rocker arm. The space is measured at a certain point in the rotation of the engine for each cylinder, basically when the push rod is just about to start lifting the rocker arm to begin to push down the valve. There is a screw locked down by a nut that runs through the rocker arm and actually touches the top of the valve stem. The other side of the rocker arm, like a see-saw, is pushed up and down by a rod that is moved by the cam shaft and lifters (lifters are little spring and oil filled cans close in size and shape to a "c" battery that actually ride on the cam).
That space is measured with a gap measuring tool which is a fan of different thicknesses of thin metal shown here. Since most auhtorities recommend some pre-load, that is screwing down the adjusting screw some number of turns after contact, the measuring tools helps you determine when the screw just touches the valve stem. With a thin blade, .005 or less, in between the screw and the valve stem, screw down until the blade is pinched. That will tell you when you are right at the top of the valve stem.
The gap is adjusted by turning the slot in the screw that goes through the rocker and touches the valve stem.
Before you can adjust the valve screw, you have to loosen the nut, which is a 14mm, (most everything else is a 13mm). After you have the pre-load set or gap or lash adjusted, you have to tighten that nut back down to lock the screw into place.
As with everything else, do your research, take your time and walk away if you get frustrated. 45 minutes of someting else resolves and avoids a lot of problems.