Question

if combining atoms have nearly similar electronegativities the bond between them is covalent

Answer 1

cuz they will tend to attract lone pairs with the same electromagnetic force(by definition of electronegativity) thus the bonding pair will be equally shared between the atoms which by definition is a covalent bond

Answer 2

it is a special type of covalent bond between two atoms by which the atom which is more electronegative attracts more electrons from the bonded pair than the other atom.