Lucas first explains three words that all mean “necessary”. It means that you need something, and if you don’t have it there will be a problem or something will just not work. These words are: ”vital”, “crucial”, and “critical”. They all mean that you need something or something must happen and if it doesn’t happen there’s a big problem. So, let’s say, for example, you want a government to be a democracy. In a democracy you need honest news and if you don’t have honest news it’s not really a democracy. If the government controls the news, then it controls what people think and so this is not by definition an honest democracy. Democracy happens when people can find out information and choose themselves who will be in the government. So, in a democracy honest news is vital, it’s crucial and critical. You must have it if you want a democracy.

Check out: How Important Is English Grammar for Your TOEFL and IELTS Scores?

Most important, principal 

The next three words mean “main”. It’s not quite the same as necessary. It means the most significant, the largest and the most important because of its size or its place. These words are “core”, “principal”, and “foundational”. These are often used for ideas and concepts that are the main parts of a larger idea. Let’s say, for example, most religions in the world believe in some kindness to other people, in being good to them. So, these religions all have some core/principal/foundational belief in kindness. That belief is a principal part of the religion, or a core part, or a foundational part of it.

Check out: TOEFL Writing Tips

Do your best to remember those words, because they will most probably play an important part in your TOEFL exam.