As water heats in a pot on a stove, it might produce some mist above its surface right before the water begins to roll. what is happening?

As the liquid water gains energy in the form of temperature, the kinetic energy of the particles increase, this causes that some bonds "break".

Now, when this happens in the top of the water (where the pressure is smaller) some of the particles can "escape" the water, and form this "mist" that you can see, this is because actually the temperature of change of phase depends on the pressure, so in the top of the water there are less pressure keeping the particles in place, and they need smaller temperature to change of phase.