Some drugs bind better to a given receptor than others. The tendency of a ligand to bind a receptor is referred to as affinity. Usually the natural ligand is the agonist with a high affinity, but synthetic drugs may bind even better.
Beware: high affinity binding of a ligand to its receptor does not automatically mean, that the receptor becomes activated.
In practice high affinity means that a ligand will bind the receptor already
The Ka can be used to define the affinity. Plotting different ligands of a receptor in a binding curve enables you to compare the different affinities. The more the curve is placed to the left (red curve), the lower the Ka and the higher the affinity. The green curve has a high Ka and thus lower affinity.
Check out the animation tab to see the effect of different affinities in the dose response curve.