A great way to think of cross training is to imagine how you spent your summers as a child. I remember in the mornings we would be playing street hockey, in the afternoon we would be playing basketball, and as a finisher all of us would finish the night off with a game of tag (which we called "freedom) before the street lights came on. That's cross training in a nut shell; it's doing different activities to get (or stay) in shape. As adults many of us get into one style of training and stick to it because we either love it or we're really good at it. This can become problematic down the line if you never switch things up, your body will get use to the same stimulus and stop adapting, and worse yet, you will begin to get overuse injuries. Think of an overuse injury like this: Take your palms and rub them together really quickly for thirty-seconds. I know most of you wont do that for that long so let me explain what happens. The continuous rubbing will cause a lot of friction and start to heat up your palms, this heat will become very hot and if you could deal with it and continue, your skin would start to breakdown. Your hands will eventually blister and hurt...
If someone is training for a marathon they think that the only thing they should do is run to prepare for the upcoming race. This is partially true, but depending on many factors the sheer volume of miles per week with nothing other than running will lead to the bodies eventual breakdown, and injuries are sure to follow. Same goes with OCR (obstacle course racers), doing a ton of hill sprints and burpees will prep you for a mountain, but without other avenues of training overuse injuries are coming. Don't think i'm picking on runners, the same goes for weight lifters; if you're doing nothing but bench pressing and back squatting all day and nothing else, your body will eventually start to fall apart.
Ideally, when you're training for a run, a race, a swim or just for life, switching up your training will not only keep you healthy but it will make you better for your preferred style of training. Training styles carry over to other training styles. If a swimming weight trains properly, they're gong to become a better swimmer. If a runner rides a bike as part of their training, they will prolong their running career by the simple fact that they are still getting their training in, but the choice of exercise is different, and this will lead to a different kind of stimulus with no overuse injuries. If a weight lifter takes up hiking, he will give his body a "rest day" that allows him to still have all the benefits of proper recovery when he goes back to weight training.
Think of different modalities of training the same way you want variety in your daily nutrition. Everyday we want to eat an array of different types of fruits, vegetables, complex carbs and different meats and proteins. If you eat grilled chicken and broccoli three times a day, seven days a week, we will be sick of it quickly. This will ensure that you're missing out on some nutrients and you will be miserable.
Even at the highest level of athletics pro athletes do this exact thing. Most NFL players during training camp spend HOURS in the pool swimming and on assault bikes for training and active recovery. You may ask "How does Swimming and Biking help a guy in the NFL?" It helps tremendously! These guys work their tails off year round, so to have them come in and beat them into the ground with more Heavy Weights and thousands of tackles a day would be downright stupid and would be a recipe for injury and overuse injuries.
Don't be afraid to switch things up...
Circuit Weight Train
Run Up Mountains
Take long slow walks on Flat surfaces
Wear a Weight Vest and Hike
Throw Kettle Bells Around
Carry Logs around in the Park
Time Your Mile Run
Bear Crawl up A Hill 10X
Shoot a Gun (stress relief/adrenalin)
Take Days OFF and Read A Book
Foam Roll for 20 Minutes