All it states is that the energy transfer or energy flow into a system is equal to the heat flow into the system plus the work done on the system. What it basically says is that a transfer of energy amounts to two types of transfers. Transfers due to heat or transfers due to work. Okay, no other transfer of energy exists. So let's see what heat engines are. Heat engines are basically systems or mechanisms that convert one form of energy into a second form of energy, namely heat into work.