A store detective is a person who works in a store and helps to catch people who might steal things without paying for them. They are like the police officers of the store, but they only work in that one place.
Their job is to watch people who come into the store very carefully, looking for anyone who acts strangely, walks around without buying anything, or tries to take things without paying. They also check security cameras to make sure they catch everything that happens.
If they see someone who might be stealing, they might follow them secretly to make sure. They might also talk to them and ask them questions to see if they can figure out if they are stealing or not. Sometimes they might even search someone if they think they have taken something.
Store detectives make sure the store stays safe and that people who do the right thing by paying for things aren't made to pay for the people who break the rules by stealing.