In the picture, graphs show the constructive interference of two 1s atomic orbitals. My question is, what does the horizontal axis in wavefunction graph represent, does It represent the distance from the nucleus? We see the pick point of wavefunction on H atom, does It mean that highest probability of electron density is on the nucleus, which is impossible. I am quite confused about the meaning of a wavefunction, can anyone help me?
You are mostly right. The horizontal axis represents the distance from the nucleus. The maximum of the electron density of 1s orbitals is in fact at the nucleus. This is not impossible, remember that electron density can be interpreted as the probability that an appropriate experiment will detect the electron there. In fact, electron density "at" the nucleus is one of the major ways NMR spectroscopy works.