An inquisition is a severe, harsh interrogation or questioning. When there's an inquisition, someone is doing everything possible to get to the bottom of something.
While inquisitions are never a very positive experience, this word has even worse connotations thanks to the Spanish Inquisition, which hunted down and persecuted anyone who didn't believe in Catholicism. Today we use inquisition to describe any serious investigation, usually an illegal or immoral one. If your mom keeps hounding you with questions about school, your job, your love life — you might yell, "What is this? An inquisition?"