The ID50 value is the concentration of the drug that binds 50% of the maximum binding to that drug's target receptor. The ED50 value is the concentration of the drug that produces 50% of the maximum response (as compared to binding). These are often different especially for agonist drugs, which has given rise to the concept of spare receptors because many agonist drugs have an ED50 that is much less than their ID50.
In general, doctors don't need to know these terms, but if you wanted to check what they are you can look in the IUPHAR database ( http://www.iuphar-db.org/index.jsp ) and then search for your specific ligand (drug - generic name) or look up its target receptor where you'll see a list of agonists and antagonist ligands (eg. for 5-HT1A receptor - http://www.iuphar-db.org/GPCR/ReceptorDisplayForward?receptorID=2310 ).
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