Sir ,Can you explain the highlighted line in figure.

According to Molecular orbital theory an electron in an atom is described by wave function (psi),which is a solution of Schrodinger wave equation.When two atoms A and B which have atomic orbitals described by wave functions A and B approach each other,their electron clouds will overlap,then the wave function for the molecule(AB ) can be obtained by a linear combination of the atomic orbitals:-
AB = N(A + B)
where,N is a normalizing constant chosen to ensure that the probability of finding an e- in the whole space is unity.Combination of pair of atomic orbitals A and B,give rise to a pair of molecular orbitals g and u.The number of molecular orbitals produced are equal to the number of atomic orbitals involved.The function g leads to increased electron density in between the nuclei,and is therefore a bonding molecular orbital.It is lower in energy than the original atomic orbitals.Conversely u results in two lobes of opposite signs cancelling and hence giving zero electron density in between the nuclei which creates a node between the nuclei.This is an anti-bonding molecular orbital which is higher in energy.Thus node creates describes that the probability of finding the electron is zero there.Such a node is absent in case of bonding molecular orbital as electron density is not zero ,in fact it increases between the nuclei.