Perhaps the most important thing to understand about Heidegger and Being is this:
The philosophical tradition before Heidegger was based on the so-called Cartesian division of thinkers, subjects; and the external world of things, objects.
Heidegger says we don’t primarily look at the world like this, i.e. through a “glass window;” rather we are there, part of it. He calls this sense of being in the world from the German, Dasein (being-there). We are part of the world which is already a pre-given for us; we, for the most part, act as others do, and we rarely think about what we are doing.
That is Heidegger’s concept of Being in a nutshell.
My own view is that Heidegger’s notion of Dasein is correct but an irrelevant dead end in philosophy. Cartesian still has rich themes to be worked.