That's Chemistry...
But basically it's just the number of electrons 'shared' between two atoms. A single covalent bond has one electron shared (e.g any diatomic bonding of the Group 7 elements)
Double covalent bonds have 2 electrons shared (e.g in the bonding of Carbon Dioxide)
Triple covalent bonds have 3 electrons shared (e.g in the bonding of diatomic Nitrogen)
Move covalent bonds make the molecule stronger