“Boil and Pop” refers to the act of immersing a plastic figure in boiling water. After about 45 seconds, the softer PVC plastics (usually used in heads and limbs) will soften enough that they can then be “popped” out with a good strong tug. Just to be safe, I use an old pot that won’t be used to cook food.
I only recommend this is as a last resort. Boiling an entire figure can weaken plastic in the long run. Also, unless you plan to paint the whole thing, boiling can discolor some plastics and factory paints.