Electronegativity describes how well an atom hogs electron - in other words, an atom where electrons want to spend more time around it.
The strength of an atom's electronegativity is determined by its bond polarity. There are different types of covalent bonds - polar or non-polar covalent bonds.
In polar covalent bonds, there is an unequal pull of electrons where one atom have more electrons around it, increasing its electronegativity. In non-polar covalent bonds, electrons tend to spend an equal amount of time around each atom.
However, as a general rule, electronegativity increases as you go from left to right across the periodic table. It decreases as you go down the periodic table.