What does that mean for a physical system within its surroundings that the energy of the universe is constant? It means that as the system exchanges energy with its environment through heat flow or thermodynamic work (pressure-volume work), there must be a corresponding loss or gain to the internal energy of the system. Think about it. This is just common sense. If heat is flowing into a system at constant volume (no work), the internal energy of the system must be increasing. If heat is flowing out, the internal energy of the system must be decreasing. If the surroundings perform work on the system and heat flow is prevented, the change in internal energy must equal the work that is being done on the system. Usually there is some combination of heat flow and work occuring in a thermodynamic transformation. Sum it up (keep the plus and minus signs straight) and the internal energy change will equal the total. The first law of thermodynamics is the easy part of thermodynamics.
Whether an MCAT passage deals with a steam engine or a phase diagram, if an MCAT passage has a theme directly concerned with thermodynamics, there will be a question or two requiring you to reason from the 1st Law. That is important enough, but it is trivial compared to the significance of understanding thermodynamics for understanding science. Stick with it here. Really concentrate. When we move through the chemistry and into biochemistry later in this course, there will be a big payoff in your conceptual understanding if you really push at every stage in the thermodynamics now to understand in a concrete, intuitive way.