Lone pair means those pair of electrons which are not forming any bond with any other atom. For example, say a compound NH3, here in the central atom i.e. Nitrogen if you see you will observe that it is forming bond with three hydrogen atoms by sharing three electrons, but we also know that nitrogen contains five electrons at its outermost shell, that means nitrogen could have form five bond by sharing five electrons but here in this case only three electrons are only used to form bond with hydrogen and still two electrons i.e. a pair of electron is left as such. This pair of electron which is not taking part in bond formation will be called as lone pair and those which are forming bonds will be called as bond pair.

For Example:

In H₂O, there are two bond pairs and two lone pairs on the central atom (oxygen).