Pressed....what and why?

Hey Im new here and just wondering.......I see a lot of descriptions saying "pressed" but what exactly does that mean?
I am assuming it suggests thats its not manufactured by a big Pharm company but then who is manufacturing it and why would that be desirable?

pressed equals someone has a lot of powder of the chemical with cutting agents, potentially, and all that, but it gets put into presses that are made to look like bars matching a real or not real bar. With mixed qualities based on QC and the press themselves