Imagine a bond between A and B. Electron density can exist anywhere, in theory, around the atoms. If the electron density is found between A and B, then it helps pull them together and hence strengthens the bond. If the electron density is found outside of A or B, then it helps pull the atom away from the bond and hence weakens it.
That's a simplistic explanation. Atomic orbitals are much more complicated than that, but it's the same general idea. Unless you get deep into quantum mechanics, you won't really get an answer to your question.