Heat is a new kind of "power" in IC². It can be produced by Heat Generators like the Electric Heat Generator, and consumed by machines like the Blast Furnace. It must be transmitted via direct contact from the producing machine to the consuming machine, as there is currently no way to store or transmit heat energy.
hU/t is a term for how many Heat Units are delivered per tick to a Heat Consumer.
A connection between a Heat Producer and a Heat Consumer takes place if both heat connections are touching each other. There are currently no "heat cables" to transmit heat over longer distances.