Indriya-samatta: 'equilibrium, balance, or harmony of abilities', relates to the 5 spiritual abilities: faith, energy, awareness or mindfulness, concentration and understanding see: indriya 15-19. Of these there are two pairs of abilities, in each of which both abilities should well counter-balance each other, namely: faith and understanding saddhā paññā on the one hand and energy and concentration viriya samādhi on the other. For excessive faith with deficient understanding leads to blind belief, whilst excessive understanding with deficient faith leads to cunning. In the same way, great energy with weak concentration leads to restlessness, whilst strong concentration with deficient energy leads to indolence. Though for both abilities in each of the 2 pairs a balanced degree of intensity is desirable, awareness or mindfulness should be allowed to develop to the highest degree of strength.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.