NCl3 is a slightly polar molecule. This is because nitrogen has a lone pair of electrons that repels the bonded electron pairs of the N-Cl covalent bonds, thus giving the molecule an asymmetric structure where the polarities of the bonds do not cancel each other out.
Nitrogen and chlorine have similar electronegativities, making individual N-Cl bond essentially non-polar. However, in NCl3, nitrogen uses only three of its five valence electrons to form bonds with chlorine. The lone pair of electrons on the nitrogen atom repels the bonded electron pairs away from itself and toward the chlorine atoms. This makes the molecule slightly polar due to the asymmetrical arrangement of the three N-Cl bonds.