"a drug interaction may occur when one drug potentiates or diminishes the action of another drug." what does this mean?

It means that in a subject who is taking two differently prescribed drugs, the drugs could have an interaction in the body. One drug could reduce the efficacy of the other or, on the there hand, could compliment. In the former case, the drug will not be effective against the disease it was proscribed. In the later case, the drug interaction could be more effective than the use of stand alone drug

Sorry its not enough info