We have two types of photoreceptors in our eyes: rods and cones. Cones come in three basic types: mostly red-sensitive, mostly green-sensitive and mostly blue-sensitive. (That's why we have Red-Green-Blue, RGB, monitors and TVs. Combining these three colors at different intensities is perceived as different colors of light.) Rods are 100 times more sensitive to light, but don't register color. Both rods and cones become desensitized in bright light, and in dim light--if they have not been desensitized--only the rods are sensitive enough to work. One of the several proteins involved in vision--rhodopsin--in rods is not sensitive to red light. So, we can use bright red light that is perceived by the red cones to illuminate things reasonably well without desensitizing the red-insensitive rods we need to see in very dim light. In other words, red light helps preserve night vision (the ability to see in very dim light) because it doesn't desensitize rods the way higher-frequency colors of light do.
Using a red map light lets you use lots of light to read the map, then use very little light to hike the trail, see the road or stumble to the bathroom.
By the way, I am a Scoutmaster, and I encourage my Scouts to use the dimmest possible setting on their flashlights, and to be careful not to shine them in the eyes of fellow night-walkers. People are often surprised at how well they can see if they let their eyes adjust to the darkness, how quickly they can lose night vision with a burst of bright light (including the reflection of high-beam flashlights off trees or buildings or fellow campers), and how long it takes to recover. It takes five minutes to recover the majority of night vision, and almost an hour to fully recover it. When we are stargazing, I use a dim red light, and have people notice the difference in how bright the stars seem when we start, after five minutes, and after 20 minutes. Some people never see many stars, because they never let their eyes adjust enough to let the starlight register.
If you have ever woken in the middle of the night to go to the bathroom, you may have noticed how much brighter the path seems than when you went to bed. That's because the proteins responsible for night vision have had a chance to return to full working order.
Hope this helps!
Joe Gill