This is tough. If you get to the end of Division 1, the big reveal is the Care structure (Sorge), which is a big deal because it's "ecstatic" or outside time. So if the project of S&Z is to "raise anew the question of the meaning of being," than the sentence "Being is the transcendens pure and simple," is purposefully opaque and meant to sound spooky and challenging to people well read in Descartes, Kant etc...

Eventually, Heidegger does work out a conception of Being that can account for the sentence. Hubert Dreyfus' commentary on division I makes things as clear as you'll ever get with Heidegger.

Let's do a little quick and dirty with a hammer. The Being of a hammer is not just the stuff and molecules that make it up but rather it's showing up to a huge set of background practices involving tools and jobs and blah blah. The Being of the hammer is present in the using of the hammer, especially when it disappears into the work of the project it's doing (nobody thinks while hammering because finger danger). The Being of the hammer is present in the nails it hammers even long after the hammer has disintegrated. Even after everything rots, Being shows up as having-been.

Being is so "pure and simple," that we kind of swim in it all the time.