Okay, so imagine you're playing hide-and-seek with your friend. You both agree to hide in different places in your house, and whoever finds the other person first wins. Now imagine that while you're hiding, someone you don't know comes into your house with a dog and starts sniffing around. They don't have permission to be there, and you don't know who they are.
That's kind of what happened in Florida v. Jardines. The police went to a house with a drug-sniffing dog, and had the dog sniff around the front door without permission from the owner. The dog then signaled that there were drugs inside the house, and the police used that as evidence to get a search warrant, which they used to find drugs inside.
But the Supreme Court said that the police weren't allowed to do that. They said that the front porch of a house is kind of like the front yard, and is a public space. But the police have to have permission to come into your private house, and using a dog to search without permission is like cheating in a game of hide-and-seek. It's not fair, and it's not legal. So the evidence of the drugs they found couldn't be used against the person who lived in the house.