When you walk towards the wind (blowing in your face), you are upwind (or going against the direction of the wind). When you walk with the wind blowing on your back, you are downwind (going with the direction of the wind).
When hunting you want to walk upwind, so you keep the wind in your face. That way animals like deer won't be able to smell you as you get closer to them.
If a deer is downwind from you, it will be able to smell you, since the wind is blowing your smell towards the deer. If he's upwind from you, he won't be able to smell you, since the wind is blowing your smell behind you (not towards the deer).
The smoke from a campfire, along with your scent will be carried by the wind and float downwind from the campfire.
Upwind is when wind blows directly in your face.
Downwind is when wind blows on your back.
--------------------------------
You may be interested in The Lost Art of Reading Nature's Signs: Use Outdoor Clues to Find Your Way, Predict the Weather, Locate Water, Track Animals and Other Forgotten Skills