Nitrogen phosphorus and potassium are considered the most essential nutrients (or macronutrients).

When reading a nutrient bag or bottle you will notice three numbers (0-0-0) these numbers represent the total value of each macronutrient in that specific product.
When looking for the right nutrients for your garden it is important to not only take the NPK into consideration but also what content of micronutrients are present, micronutrients include calcium (which is almost a macro it is so important) iron, cobalt, chlorine, copper, molybnium, manganese etc, all of these micronutrients play an essential role in keeping your garden healthy, but as always adding too much of anything can lock out something else so everything needs to be added in moderation.