In any area of expertise, a fundamental is a core, foundational theory from which more complex topics are derived. Everything else is based upon and dependent on these ideas. While any path of study will eventually lead one back to the fundamentals, it helps students if they are covered early and classes "scaffold" up to higher-level topics.
When it comes to hands-on technical skills, a key milestone in one's studies is achieving automaticity with each level of its fundamentals. Meaning that the goal with a fundamental is not just to achieve conscious mastery (a logical understanding of the theory) but to achieve unconscious mastery (a deep familiarity that no longer requires logical thought). This allows you to pursue more complex ideas that use that fundamental skill in peace. If you don't reach this milestone, it's deeply exhausting to apply the things you're learning in practice, and even simple tasks can lead to burnout.
Aside: Although many art classes try to skip it in the guise of "pure intuitive training," true automaticity generally cannot be achieved without first achieving mastery over the theory. That mastery gives your brain the formula it needs to skip the hard parts later. Automaticity is also best trained at each minor stage to minimize overwhelm. If you imagine yourself reading a textbook about, say, lighting skills, then automatic training should begin after each sentence or paragraph is digested, NOT after entire chapters or volumes pass by. But we'll talk about this more elsewhere.
In art, we can understand the fundamentals to be the underlying theories that apply to even the simplest of subjects, to works with no subject at all, and to the overall experience of learning and creativity. (A subject is the "thing" you are drawing, painting, etc. Abstract art lacks this.)
For example, if I described how a camera was looking at a particular arrangement of children's building blocks with words, someone who understands and has practiced the fundamentals of constructive drawing sufficiently could position everything accurately in space and render it photorealistically with only the given information. And they could do so quickly and confidently with little thought.
When we jump right into learning how to draw flashy things like 3D comic book characters and intricate environments, the complexities of everything else involved in depicting them overwhelms us and obscures the basics. We become preoccupied with these more niche, higher-level topics, like human anatomy or architecture, but then can't even use what we've learned because we have no foundations to apply them to.
Training these skills doesn't need to be boring, and if it's scaffolded properly, it shouldn't interfere with your ability to make complete works featuring things that interest you from the start. Learning the fundamentals doesn't mean you can't design characters or tell stories or do other fun things with your art. However, we may have to break those ideas down further and lower our expectations a bit in terms of the style and approaches we use early on.