Instability Training is a broad term that includes any type of exercise performed while unstable. This could include the use of props like a BOSU or TRX Bands, performing exercises while balancing on one leg or arm, or over loading one side of your body with weight to inhibit your balance.
Instability Training can benefit your workout in many different ways including; warming up your muscles, progressing an exercise, and increased muscle recruitment. There are hundreds of instability exercises but some of my favorite are listed below:
Single Leg Squats