Ka and Kb stand for acid dissociation constant and base dissociation constant. Simply put, Ka describes how acidic a solution is, and Kb describes how basic a solution is. Every solution has a value for both Ka and Kb.
It's important to note that Ka and pH, while they describe similar things, aren't the same. In the same manner, Kb and pOH describe different things. The most common use of Ka and Kb are in equilibrium equations. For the acid HA, Ka is always equal to the product of the concentrations of hydronium and conjugate base ions divided by the concentration of the acid (Ka = ([A-][H+])/[HA]).