The 1st Law of Thermo is really just restating that bigger law, Conservation of Energy, in terms of thermal quantities like Heat, Energy, and Work.

Heat, Energy, and Work sound like vague terms that have no specific meaning. But in thermodynamics, they have a specific definition, and they can be QUANTIFIED exactly. That means if I give you a cup of hot water, and you take a bunch of very careful measurements over time, using some math formulas it is possible to tell exactly how much Energy the water lost while it was cooling off. Units of Energy are called Joules.

To emphasize that these terms have a specific definition, I capitalized them. So when I say “work” I mean like doing the dishes, clean your room, blah blah blah. But when I say “Work” I mean something exact. Units of Work also have the unit of Joules, just like energy. To understand what the exact definitions are, see the Wikipedia entry for “Joule”.

So basically the meaning of the 1st Law of Thermo is this. Energy cannot be magically created, nor made to disappear. For any thermodynamic system, if you add up the quantities of Heat, Energy, and Work, it is a constant number. The sum will NOT CHANGE unless you add or remove one of those 3 things. That means there are only THREE WAYS IN OR OUT of the system. If your glass of water cools off, its temperature goes down, and therefore it’s energy goes down. But the 1st Law says you have therefore done some Work, because that energy cannot magically disappear. It has to GO SOMEWHERE. Where did it go??? In that example, it went into the air surrounding the glass. If you add up the new Energy that air has, it equals EXACTLY the energy lost by the glass.