"Heat" is a metaphor. When rubbing two sticks to make a fire, the sticks will get warm before they catch flame. Similarly, before attaining a direct experience of emptiness on the path of seeing, one has intimations on the path of juncture, and so one of the stages on the path of juncture is called heat.
The best discussion of the stages of the path that I've seen in in Chapter 7 of Khenpo Karthar's book, Dharma Paths.
Having obtained the supreme freedoms
and conjunctions of the precious human existence,
endowed with faith, energy, and intelligence,
Having attended on a worthy spiritual friend
and received the pith of the holy instructions,
May we practice these properly, just as we have received them,
without obstacle or interruption.
In all our lives, may we practice and enjoy the holy dharma.