Finally installed the sway bar today that has been sitting in my garage for a year or more. I set the adjustable heims the same distance as each other and bolted them into the u tabs.

Can someone please explain preload to me? Does it have to do with one drop link being longer than the other?

Yes. If you drive the car often alone you may want to put one slightly longer than the other. Put your weight in the seat and check to see if one sides slightly higher than the other.
Any bushings in your Hiem joints? Without them you could eventually see some cracking around where the sway bar goes through the chassis. Might be fine for AX but for the street you need something to absorb the vibration.
B

Preload is....

If you remove the drop links all together, then lower the car, then measure between the a-arm and the sway bar ends you'll often find that each side measure differently. In order to remove all tension from the sway bar when unloaded, you may need different length (or better, adjustable) drop links to account for this difference. The bottom line is that you want the sway bar to be neutral until acted upon by outside forces. A preloaded bar is not neutral.

With the car on the ground, your weight in the drivers seat, half a tank of gas both droplinks should not be loaded.

With adjustable droplinks this is easy as you can turn them by hand until you don't feel any resistance and the link rotates freely.
Tighten the nuts and you're done.

Andy