Excellent article by author Patricia Wrede on the subject. Particularly pertinent is this passage (bolding is mine):
The first problem I nearly always run into when I’m arguing with someone about this is that they don’t understand that “Jake stumbled out of bed, shut off his alarm, and sleep-walked through his morning routine” counts as “showing” just as much as saying “Jake stumbled out of bed in the general direction of the alarm. He got the alarm shut off after three tries, then shuffled into the bathroom. He turned the shower on and brushed his teeth while the water warmed up. He had time for a longer one than usual this morning, which almost made the damned alarm worthwhile. He was contemplating, in a groggy sleep-soaked fashion, whether to shave or pretend for the rest of the day that he was growing a beard, when the scent of coffee penetrated to the bathroom.”
The two descriptions have different levels of detail, but they are both “showing” what Jake is doing in the morning. The “telling” version is “Jake had a hard time getting up in the morning.”
In other words, “telling the reader” means giving the reader the conclusion they would draw, without giving them any of the actions or thoughts or descriptions that would lead them to that conclusion.