Runout referres to the concentricity of your ammo. Or straightness.

If you roll your ammo on a mirror, do you see the tip of the bullet wobble? Thats tip runout. they sell concentricity gauges to measure it. You can roll your ammo on the gauge and measure the runout at the neck, at the base of the bullet, at the tip.

Alot of things can contribute to runout. Varying neck thickness for one. You can get tools to turn your necks, but I highly recommend against it. I went down this road, and its painful, time consuming and in the end, just cull your brass. Other things that contribute is your decapping expander ball. Does it sit straight in the die? Lack of inside neck lubing can contribute. When you drag your expander thru if one side slides thru easy you can get runout. I find the shell holder to be a big culpit. The slop in a shell holder when you push a piece of brass in can make a bullet seater die not load your bullet straight. Anymore I let my shell holder/brass float.