can you just explain me the (n+l) situation.

l here means the azimuthal quantum number( if l = o it means - s orbital, l = 1 means p orbital and l = 3 means d orbital)

 

say we have 4s orbital and 3d orbitals ;

In 4s the (n+l) would mean 4 + 0 = 4

In 3d it would mean 3 + 2 = 5 

the entity with the lower answer has lower energy and is filled first.

Thus 4s is filled before the 3d orbitals.