Principal energy levels are made out of sublevels, which are in turn made out of orbitals, in which electrons are found.
An atomic orbital is defined as the probability of finding an electron in an area around an atom's nucleus. Generally, orbital shapes are drawn to describe the region in space in which electrons are likely to be found. This is referred to as "electron density."
Covalent bonding occurs when two atomic orbitals come together in close proximity and their electron densities overlap. The strongest type of covalent bonds are sigma bonds, which are formed by the direct overlap of orbitals from each of the two bonded atoms. Regardless of the atomic orbital type, sigma bonds can occur as long as the orbitals directly overlap between the nuclei of the atoms.
Single covalent bonds occur when one pair of electrons is shared between atoms as part of a molecule or compound. A single covalent bond can be represented by a single line between the two atoms. For instance, the diatomic hydrogen molecule, H2, can be written as H—H to indicate the single covalent bond between the two hydrogen atoms.