CISK, or "Convective Instability of the Second Kind", is a popular theory that explains how thunderstorms can evolve and organize into hurricanes. CISK is a positive feedback mechanism, meaning that once a process starts, it causes events which enhance the original process, and the whole cycle repeats itself over and over. Below is a video explanation of CISK.
Latent heat is simply heat released or absorbed by a substance (in this case, water vapor) as it changes its state. When water vapor condenses into liquid, it releases this heat into the surrounding atmosphere. The atmosphere around this condensation then warms.
Since warm air is less dense than cooler air, the warmer air takes up more space. This expansion of this air (red arrows) forces more air outside away from the center of the storm and the surface pressure (which is the weight of the air above the surface) decreases.