It means that if the unit level is 0 then XP gain is 0 too?The xp for a given unit is determined by the units level. higher level is worth more xp.
If you want to change how many XP you gain, go to 'Advanced' --> 'Gameplay Constants' and scroll down to 'Hero XP Gained'. There you can change the base XP gained, bonus XP per level of killed opponent and other stuff to change XP gaining. Modify to your wishes!
To be able to change the default values to the values you want, check the box 'Use Custom Gameplay Constants'