What is Exchange current density, transfer coefficient, standard rate constant?

At equilibrium the anodic and cathodic currents are equal and opposite; their common magnitude per unit area is the exchange current density $j_0$. The transfer coefficient $\alpha$ ($0 < \alpha < 1$) is the fraction of the electrical energy $nF\eta$ that lowers the barrier of the cathodic reaction. The standard rate constant $k^\circ$ is the common value of the cathodic and anodic rate constants at the formal potential $E^{\circ\prime}$.