With mechanical systems, we determined that you could change the energy of a physical system through the work done on the system by non-conservative forces.  In thermodynamics, heat gives us a second way to transfer energy into or out of a system.  You can change the energy of a thermodynamic system by doing work on the system and by transferring energy into or out of the system due to temperature differences between the system and its surrounding environment.  So how do we keep track of how the energy of a thermodynamic system is changing?  By using the First Law of Thermodynamics.  The First Law of Thermodynamics lets us keep track of the energy that flows into or out of a system through two processes, work and heat.