“Reasonable articulable suspicion”, is the official way of saying that.

“A good explanation” is very undefined. The police has to have reasonable suspicion that the person has committed a crime, and they have to be able to articulate, ie explain that said reasonable suspicion of having committed a specific crime.

They just make it up all the time though, but most of the cops don’t even seem to know the law. They just do what other cops do. And never have to take responsibility for breaking the law.