If you have a hydraulic lifter in your engine, setting the zero lash is critical to ensuring proper valvetrain function. The zero lash is the point at which there is no clearance between the rocker arm and the valve tip. This is typically accomplished by screwing the adjusting nut down until it makes contact with the rocker arm, and then backing it off a specified amount.
For example, on a Chevy small block with 1.6:1 rockers, you would back the adjusting nut off a 1/4 turn. This may seem like a small amount, but it's critical in ensuring that the valvetrain operates correctly. If the zero lash is not set correctly, it can lead to a number of problems, including premature valvetrain wear, noisy operation, and reduced performance.
If you're not sure how to set the zero lash on your hydraulic lifters, consult your engine builder or a qualified mechanic. They'll be able to properly set the zero lash and ensure that your engine is running optimally.