The activity or sport of riding in a boat that's propelled by the wind is sailing. If not even the slightest breeze is blowing, it's not a great day for sailing.
Sailing can be as simple as jumping aboard a friend's sailboat and taking a ride on a lake. Being in charge of piloting the boat, shifting the sails, and keeping it moving on the water, is also sailing. If you're curious how it's done, you can take sailing lessons!