Electronegativity is a measure of an atom’s attraction for the electrons in a bond.
Across a period from left to right the electronegativity of atoms increases.
As you move from left to right across the periodic table, atoms have a greater nuclear charge and a smaller covalent radius. This allows the nucleus to attract the bonding electrons more strongly.
Going down a group, the electronegativity of atoms decreases.
As you move down a group in the periodic table, atoms increase in size, with a greater number of energy levels.
The extra energy levels and increased covalent radius keep the bonding electrons further away from the nucleus.
This screening effect is caused by the extra energy levels and means that atoms further down groups have less attraction for the bonding electrons.
Both of these trends show that fluorine is highly electronegative (it pulls a shared pair of bonding electrons towards itself).