Equivalent is called the particle which is chemically equivalent (is equivalent) in the acid and main reactions to one ion of hydrogen, and in reactions of oxidation-reduction type - to one electron. The equivalent is expressed by number without dimension whereas equivalent weight is measured in g/mol.
The substances entering chemical reaction undergo changes of structure and the building, turning into reaction products. Concentration of initial substances decreases up to zero if reaction goes all the way. But the return reaction when products break up to initial substances can proceed. In that case balance when the speed of direct and return reaction becomes identical is established. Certainly, equilibrium concentration of substances will be less initial.