Heidegger was aware of the fact that seizing what is essential is a minority rather than a common ability. For, he asserted, it is clear today that the more people talk and write the less they have something essential to say.
Which begs the question of what we mean by 'essential' and what it means to be talking about 'essential' things.
The word essential comes from the Latin verb esse, to be, and to say something essential is to describe a characteristic of Being, i.e. being able to seize the being of a thing in its relationship to Being.
In other words when you ask questions such as 'what is mathematics', 'what is truth', 'what is overthinking?' you are laying the brick work for an answer which will consider what something is in its relationship to Being, that which is.
And that which is is truth.
And, of course, by asking what is essential I was seeking to give the essence, i.e. meaning, of the word 'essential'.
By virtue of saying what something is or saying something that belongs to that which is you are saying something essential.