Concentration is a gathering together of the consciousness and either centralising at one point or turning on a single object, e.g., the Divine; there can also be a gathered condition throughout the whole being, not at a point. In meditation it is not indispensable to gather like this, one can simply remain with a quiet mind thinking of one subject or observing what comes in the consciousness and dealing with it.
One can concentrate in any of the three centres which is easiest to the sadhak or gives most result. The power of the concentration in the heart-centre is to open that centre and by the power of aspiration, love, bhakti, surrender, remove the veil which covers and conceals the soul and bring forward the soul or psychic being to govern the mind, life and body and turn and open them all fully to the Divine, removing all that is opposed to that turning and opening.
The power of concentration above the head is to bring peace, silence, liberation from the body sense, the identification with mind and life and open the way for the lower (mental, vital, physical) consciousness to rise up to meet the higher consciousness above and for the powers of the higher (spiritual nature) consciousness to descend into mind, life and body.
The power and concentration in the eyebrows is to open the centre there, liberate the inner mind, and vision and the inner or yogic consciousness and its experiences and powers.
They lived in the mind and knew truth from within;
A sight withdrawn in the concentrated heart
Could pierce behind the screen of Time's results
And the rigid cast and shape of visible things.