You can think of an atomic orbital as an invisible cloud around a single atom. When two atoms get close, their electron clouds overlap and mix together. This mixing process creates a brand new electron cloud called a molecular orbital.
The size and shape of this new cloud determine how the atoms stick together. Sometimes the mixing puts most of the electron cloud directly between the two atomic nuclei. This setup pulls the atoms together and creates a highly stable bonding orbital.
Other times, the mixing pushes the electron cloud away from the space between the nuclei. This leaves the positive nuclei exposed and they strongly repel each other. We call this an antibonding orbital because it actively weakens the chemical connection.
Students often confuse molecular orbitals with physical shells or solid planetary tracks. In reality, they are just mathematical probability maps showing where electrons probably exist. Electrons moving into lower energy bonding orbitals will release energy and stabilize the molecule.
Electrons forced into higher energy antibonding orbitals will absorb energy and make the molecule less stable. Every molecule has a unique set of these orbitals that dictates its specific chemical traits.