The first law of thermodynamics is energy conservation applied specifically to heat and work: a system's internal energy changes by exactly the heat added to it minus whatever work the system does on its surroundings. This is a precise accounting rule, not a vague generalisation, meaning energy put into a system always shows up somewhere, either raising the system's own internal energy or leaving as work done outward, with no energy quietly vanishing or appearing from nowhere along the way.
Heat and work are the only two ways energy can actually cross a system's boundary
The first law identifies exactly two channels by which energy can move across the boundary separating a system from its surroundings: heat, energy transferred because of a temperature difference, and work, energy transferred through some kind of force acting through a distance, a piston compressing a gas, say. Any energy entering a system through either of these two channels has to end up increasing that system's own internal energy, unless it's simultaneously leaving again through the other channel, which is exactly what makes the first law function as a genuine, checkable accounting identity rather than a loose approximation.
This accounting works the same way for every physical process without exception
Because the first law's energy accounting holds without exception, it applies identically whether the system in question is a car engine converting fuel's chemical energy into mechanical work, a person's body converting food energy into heat and physical activity, or a simple gas expanding inside a cylinder, doing work on a piston as it pushes outward. This universality is exactly why the first law functions as one of physics' most foundational, load-bearing principles: it guarantees that tracking energy carefully through any physical process, however different that process looks on the surface, will always balance, with input energy accounted for completely across internal energy change and work done.
What we're still unsure about
That the first law's energy accounting, heat in minus work out equals change in internal energy, holds universally across physical processes is extremely well established, among the most thoroughly confirmed principles in the whole of physics. What's more genuinely a matter of practical, ongoing engineering challenge is exactly how completely a real system's actual energy flows can be measured and tracked in practice, since real systems often have genuinely difficult-to-measure energy losses, friction, unwanted heat leakage, that make the first law's accounting harder to verify precisely in a specific real-world device even though the underlying law itself is never actually violated, and engineers continue to develop better measurement techniques to track down exactly where a given system's energy is actually going.
This sits inside First Law of Thermodynamics, one of seven topics in Thermodynamics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.