This is the uneven distribution of bonding electrons in a molecule creating partially positive and negative sides of that molecule. example: water and ammonia have this, but methane does not

Following are the molecules of interest H2O, NH3 and CH4.

In each of the above system, central element undergoes sp3 hybridization.

For example, in H2O, oxygen undergoes sp3 hydridization
in case of NH3, nitrogen underdoes sp3 hybridization,
and in case of CH4, methane undergoes sp3 hydridization.

Followed by hybridization, the hybridized orbitals are unsymmetrically occupied in case of H2O and NH3. In water, two hybridized  orbitals are occupied by H atom and remaining two orbitals contain lone pair of electrons. In case of NH3, three hybridized orbitals are utilized for bonding with H atom, while one hybridized orbital contain lone pair of electron. In contrast to this, in case of methane all the four hybridized orbitals are utilized for bonding with hydrogen. Thus, methane is symmetrically substituted in contrast to water and ammonia. Due to this,  there is the uneven distribution of bonding electrons in H2O and NH3, thereby creating a partially positive and negative sides of that molecule.