Anto bonding is a type of chemical bonding. Anti bonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei. The overlap of constituent atomic orbitals is said to be out of phase and the electrons present in each anti bonding orbitals are repulsive and act to destabilize the molecule as a whole.
H2- is a diatomic molecule containing 3 electrons. One of the electron found in anti bonding orbital. Hydrogen atoms have a single 1s electron. The 1s orbital has 2 electrons. One is spin 'up' electron, another one is spin 'down' electron. If a hydrogen atom contains an extra electron, forming a H- ions , the 1s orbital is filled.
H atom and H- ions approach each other to form a sigma bond between the two atoms. Each atom contribute the electrons to fill the lower energy sigma bond. Extra electrons fill a higher energy state to avoid interaction with other 2 electrons. The higher energy orbital is called the anti bonding orbital. ( σ * ).