[Unit experience] What are the effects?

[Brave Brave Sir Robin]

That's "attack stats" really. Both melee and missile attack ratings go up one for one with chevrons.

Yes but for missile units, apparently attack stat increases are equated to accuracy increases. So a freshly recruited unit of Cretans will be somewhat accurate while a gold chevroned unit will almost all hit with their arrows. Not sure what weapons upgrades would equate to though.