As the water is heated, it rises from the bottom of the beaker to the top. Cooler water at the top of the beaker sinks to take the place of the heated water. The movement is made visible by the coloured solution of the crystal.
When the not water rises, heat energy is carried from one place to another. This kind of heat movement is called convection.
Convection is the movement of heat energy from one place to another by the movement of the material itself.