In short: telling is outright stating what a character feels (Sam's been supposed to be back from X an hour ago, and Dean's worried.) and showing is describing it (Sam was supposed to be back an hour ago. Dean can't stop pacing through the room, runs a hand over his face repeatedly and glances at his phone every other minute.) Or if you take settings: don't say "this place is creepy" but describe WHY it is, what it looks like, how it makes the character feel.

I've been SO SURE I had a link to a workshop or two, but I can't find them right now. /o\