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. Thus, a high affinity does not automatically mean a high response.
In practice, high affinity means that a ligand will bind the receptor already at low concentrations.
The Kd can be used to define the affinity. Plotting different ligands of a receptor in a binding curve enables you to compare the different affinities. A higher Kd means that there is a higher (equilibrium) rate in which ligand becomes dissociated from the receptor. This means that more ligand is unbound. On the contrary, a low Kd indicates that most of the ligand will be bound. In case of concentration-binding diagrams, the curve of a ligand with a higher binding will be placed to the left (red curve), and will have a lower Kd-value, thus a higher affinity than the blue compound. In case of a ligand that has less affinity (thus a higher Kd), the curve will shift to the right, and a higher concentration of ligand is needed to bind to 50% of the receptor.
In short: ligands with higher affinity will be shifted to the left in a concentration-binding diagram, and its Kd will be lower.