How do dispatch boards work?

[Jumbuck]

Could someone explain to me how expediting dispatch boards work. I hear "first in first out, etc" but I don't know what all that means. If someone could explain to me as if I were a 2 year old, I sure would appreciate the help.
Jumbuck

 

[xiggi]

Regardless what a carrier says a lot changes based on distance from shipper, time until pick up, truck size, dispatcher mood, driver attitude. with some companies your acceptence rate on offered loads, etc.

But in a nutshell first in first out is...

Five sprinters who all have the same cargo space are all within a 50 mile radius from the shipper. The truck who has been setting empty in service the longest will be offered the load first.