HIV is RNA virus ,  ++SS RNA virus , the main enzyme reverse transcriptase. When copying make many mistakes when Copy RNA to DNA. 

The enzyme do not have proofreading function like human. 

This is why the mutation lead fast evolution. 

The drug target reverse transcriptase using NRTI drug (Nucleotide reverse transcriptase inhibitors).

 The drug blocks the virus from copy RNA to DNA 

The virus mutates often so the resistance can develop causing the drug stop working or not effecting.