It is rare that a drug binds to a single type of receptor. E.g. The endogenous neurotransmitter serotonin (5-hydroxy tryptamine, or 5-HT) can bind to 15 different types of receptors. Selectively active drugs have a very high affinity for only one type of receptor (also: high specificity).
In the example, ligand A is much more potent at receptor 1 than at receptor 2, normally as a consequence of higher affinity for the receptor.
Ligand B has equal potency (and normally similar affinity) at both receptor types. The drug is less specific (in jargon: it is less ‘clean’).
Highly selective drugs are difficult to design, because for most endogenous signalling molecules, more receptor types exist with similar binding pockets.