Heat causes things to expand (or get larger). Cold causes things to contract (or get smaller). Heating a object sealed inside another object can produce some interesting results.
Placing your hands around the outside of the bottle warms the air inside the bottle which then expands. Since the bottle is sealed by the penny, the heated air can only escape by breaking the ice around the rim and moving the penny. When the build up of heated air inside the bottle grows great enough, the ice gives and the penny pops off the bottle!