Some people, musicians and non-musicians, possess a “great ear.” That is to say, it takes minimal effort for their brain and auditory system to process, label or produce sounds or music that they hear. Many of these people have no way of knowing how or why they have “good ears.”
Then there are those of us that have to work diligently to consciously develop the connection between our ears and the other aspects of playing, transcribing, communicating, composing and performing on our instruments. The ear can be trained and every effort the instrumentalist or singer makes in this direction will be useful to their performance. Every intention, I should say, will be useful: for self-critical frustration in this area will not help you towards your goals.
In fact, much of the “hard work” of ear-training should be seen as quite fun because it is the equivalent of nursery school or 1st grade level activities, so accessing your inner child will make the work fun. In other words, these are things that could/should be taught (and are in some cultures) at a very early age, but most of us have no choice but to go back and piece things together: but anyone can do it, and it is a fun supplemental training to your core instrumental studies. Actually, it is dangerous to even think of ear-training as supplemental, it is also a core training, but I do have the view that it is worth delving into on its own merits (similar to Conscious Breathing) and weaving it into the other technical, physical and conceptual instrumental trainings.
The basics of music theory go hand-in-hand with the basics of ear-training, but you only need a minimum of terms and concepts before you can start training your ear in earnest. Ear-training is both very standardized and highly adaptable to individual needs, mostly because the strengths and weaknesses of different students can vary widely.