Acclimatization is the act of becoming accustomed to a change in environment. If you are planning on completing an endurance event in an environment different from the one you normally live in, acclimation is very important.
Think about the last vacation you took where you left your state. Maybe you were lucky enough to visit Hawaii or Florida. What is the first thing you noticed when you stepped off the plane? Could you feel the air?
Perhaps the humidity made you feel like you were sweating just standing still. By the time you got to your cab, you were as sticky as the underside of a theater seat.
Now, go run 26.2 miles.
An even more extreme example, and a favorite of heat training experts, is the Badwater Ultramarathon. Badwater is a 135-mile sufferfest through Death Valley in July where the temperatures can and do break 130 degrees Fahrenheit.
Runners arrive in Death Valley sometimes two weeks before the start so that their bodies can begin the difficult transformation needed to survive what probably is the most grueling race on the planet. Not doing so is to openly court failure.
But what if you don't have the funds to get to your destination race two weeks early? There are ways to acclimate without traveling:
When you normally go for a run you'll throw on a tech shirt and some shorts and head out the door. That is great if you want to be comfortable. But to prepare your body for tougher temperatures you may want to consider a long sleeve shirt, then a shirt, and running tights, then shorts. Depending on how hot you're talking about, a hat and even another top layer may be used.