Planing is when a boat goes really fast on the water and it feels like it's flying.
When the wind blows on the sail, it pushes the boat forward. The faster the wind blows, the faster the boat goes.
When the boat starts to go really fast, it leans over to one side and the hulls lift up out of the water a little bit. The boat starts to ride on top of the water instead of plowing through it.
This is called planing. It's like when you ride your bike really fast and you feel like you're gliding instead of pedaling.
When a boat is planing, it's usually going really fast and it's exciting. But it can be tricky to control, so it's important to be safe and wear a life jacket.