Bonding and Antibonding
Orbitals are defined by a set of wavefunctions. Hence, when two orbitals overlap they can either interfere constructively (bonding) or destructively(anti bonding).

Lets take two pz orbitals orbitals overlapping with each other. You must understand that the p orbitals look like a dumbbell. One side of the dumbbell is + and the other side is -.

If - meets - we have constructive overlap. If - meets + we have destructive overlap.

The former gives a bonding orbital ($\sigma$) and the latter an antibonding orbital ($\sigma*$).

Depending on the molecule it is often energetically favourable to be in a bonding state hence bonding orbitals are lower in energy to their anti-bonding counterparts.