Squeeze means tighten or flex your muscles. Focus on really squeezing your muscles. Slow exercise movements with the proper focus on squeezing can help the muscles develop more efficiently than rapid, unfocused exercises.
Abs: Crunch Kicks
Focus on squeezing your ab muscles throughout the whole exercise. Keep your back fully flat against the floor, including your lower back.
Squeeze chest muscles. Back straight, shoulders tight against body (not shrugged). Elbows tucked in. Focus on maintaining a slow and controlled movement, and feel your chest muscles working.