Many airports in the western United States are located at altitude.  In the thin air, a departing aircraft’s propeller and wings are less aerodynamically efficient.  And without a turbocharger, the aircraft’s engine won’t be able to produce full power.  All of that hurts the aircraft’s ability to climb. Unless the aircraft is handled properly, after lifting off the runway it may travel for a distance

on a cushion of air existing between the aircraft’s wings and the runway, and then ultimately crash. 

Hot weather makes the air even thinner.  Thus, in hot weather, the airplane behaves as though the airport is at an even higher altitude than it actually is.  The altitude at which an aircraft "thinks" it is operating is called the "density altitude." 

When a pilot combines a high "density altitude" with a heavily loaded aircraft, it can lead to a challenging situation.  In fact, unless the pilot is experienced in "high, heavy and hot" operations, the combination can be a recipe for disaster.  Just a few examples of "high density altitude" accidents involving heavily loaded aircraft can be found in the NTSB database here, here, and here

The airport in this video sits 1,300 feet above sea level. That’s not particularly high.  However, the temperature on the day of the accident was almost 100 degrees.  That made the airport’s "density altitude" more than 4,100 feet.  Add a heavy load and, and even with a turbocharger, the density altitude was too much for this pilot to handle.