Heidegger in 4 words: Everything isn't a thing

That's it really. Heidegger in 4 words. Five if you're pedantic. Everything isn't a thing. And if we think that everything is a thing then we use 'thing' language and think 'thingfully.' Rule number one of living an ontologically virtuous life is not to be thingful.

And (hypothetically) if there is reality that is not describable as a thing or things then if we use thing language we will reinforce the cover-up of all that reality.

And (hypothetically) if in fact it's the 'non-thing' reality that grounds all of what we think of as things, then we are in trouble as far as having any sort of adequate understanding of the world.

Hence Heidegger's dislike of ontotheology which makes God too a thing, reducing God to human thing-type ways of thinking and understanding.

In its attempt to know the world philosophy has covered up Being itself; it has squeezed the world into its own mould but in doing so has distorted our understanding of reality.

So, according to Heidegger, his phenomenological approach is the only way of access to the most basic levels of reality. Why? Because it offers the only means of avoiding thinking of the world as so many entities understood and dealt with in the ways that the history of Western philosophy has dictated.

Heidegger expresses it this way (in an uncharacteristically clear sentence, at least for those who have read some of his work):

Basically, all ontology, no matter how rich and firmly compacted a system of categories it has at its disposal, remains blind and perverted from its ownmost aim, if it has not first adequately clarified the meaning of Being, and conceived this clarification as its fundamental task. (Being and Time, 1962, p.31)