I am new on this forum and new to DWG format. What is the meaning of Blocks in DWG (for example, compared to old Microstation DGN files?

I'm not familiar with the software you mentioned in your post; however, the context in which I use blocks - they are objects that you can manipulate and add to a drawing with and with out attributes.  I have a library of blocks drawn in individual sheets and I extract those and drop them into new drawings as opposed to drawing something from scratch every time.  An example would be compasses and symbols.

Now I view program Ivan Explorer and IvanSampleR27.dwg - it seems that all drawing entities are grouped by Block.

[QUOTE=AndrzejBor]I am new on this forum and new to DWG format. What is the meaning of Blocks in DWG (for example, compared to old Microstation DGN files?[/QUOTE]
 Help [here for AutoCAD 2017, but this topic won't be much different in other versions] will tell you about Blocks.