To start off, molecular orbitals are the linear combination of atomic orbitals. For every atomic orbital you put in, you get a molecular orbital out. So if I put in 1 AO from atom A and 1 AO from atom B, I end up with 2 MO's shared by both atoms - one bonding, one antibonding.
HOMO is the highest occupied molecular orbital; LUMO is the lowest occupied molecular. Putting electrons into the LUMO takes energy - some compounds can do this with heat, others can do it with light - it just depends on the molecular orbital arrangement. I'll get back to this in a second...
Remember that bond order is defined as (Electrons in B.O. - Electrons in Anti-bonding Orbitals)/2
Bonding orbitals are orbitals whose overlap allow for the formation of a bond; anti-bonding orbitals are orbitals that, when an electron is placed into them, reduce the bond order.
The reason I'm making this distinction is because having an electron in an antibonding orbital does not necessarily mean that your bonds will be broken - it is only when the number of electrons in anti-bonding orbitals equal the number of electrons in bonding orbitals.. So if you had 2 bonding electrons and 2 antibonding electrons, your bond order would be zero. (In other words, you wouldn't have a bond.)
But if you had 4 bonding electrons and 2 anti-bonding electrons, then your bond order would be 1 - you would still have a bond (A single one, albeit, but still a bond.)
You can have bond orders of 1.5, 2.5, anywhere in between, it just depends on the molecular orbitals available to that molecule.
Hope this helps.