Nitrogen can form up to four covalent bonds, most commonly seen in ammonium. Ammonium is a positive ion, which isn't as stable as ammonia, which has only three covalent bonds.
A covalent bond typically occurs between two or more nonmetal atoms. To try and fill up their respective outer orbitals, atoms will share their outer electrons. Nitrogen in its elemental form is found with five outer electrons in a shell that could be filled with eight. When nitrogen forms three covalent bonds, the outer orbital is essentially full, and ammonia, a neutral molecule, is formed. Through various chemical techniques, nitrogen can be put in a situation where it forms four covalent bonds, which is possible because shared electrons do not physically take up space in each others' orbitals.