Describe the principle of overload, and explain how frequency, intensity, time, and type can you safely apply overload to your favorite workout.

The principle of overload states that the body adapts to every stimulus. Every stimulus needs adaption and in order for a muscle (including the heart) to increase strength,a greater than normal stress or load on the body is required for training adaptation to take place - a different stimulus is required.
Different stimulus can be doing the same workout but in smaller time frame (next stimulus can be to run 5km for 25 minutes, if you have run last time for 30 minutes), or every routine can be done more times...Also changing the workout type is way of changing the stimulus (you can incorporate in you workout routine different sports: running, swimming, hiking..for example).