What is Electronegativity?

In short

Electronegativity is a measure of how strongly an atom attracts the shared electrons in a covalent bond. Fluorine has the highest value.

In simple words

On the Pauling scale, fluorine is $4.0$ and caesium is about $0.7$ . A bond between two atoms of very different electronegativity is polar, with the shared pair closer to the more electronegative atom, and a difference above about $1.7$ is usually treated as ionic. Electronegativity rises across a period and falls down a group.

Example

In hydrogen chloride, chlorine ( $3.2$ ) pulls the shared pair more strongly than hydrogen ( $2.2$ ). Chlorine is slightly negative and hydrogen is slightly positive.