For example, amines or nitrogen-containing heterocyclic compounds have a lone pair of electrons on the nitrogen atom and can thus act as proton acceptors.
A simple explanation of the term would state that electropositive atoms (such as carbon groups) attached in close proximity to the potential proton acceptor have an "electron-releasing" effect, such that the positive charge acquired by the proton acceptor is distributed over other adjacent atoms in the chain.
The converse is also possible as alleviation of alkalinity: electronegative atoms or species (such as fluorine or the nitro group) will have an "electron-withdrawal" effect and thereby reduce the basicity.
To this end, trimethylamine is a more potent base than merely ammonia, due to the inductive effect of the methyl groups allowing the nitrogen atom to more readily accept a proton and become a cation being much greater than that of the hydrogen atoms.
Phosphazene bases also contain phosphorus and are, in general, more alkaline than standard amines and nitrogen-based heterocyclics.