The xblock dialog interprets an xblock and it's various fields.

xblock.png

At the top of this dialog is the "File Range Control". The file range control gives you a quick graphical view of where in the file this block is located.

Next comes the xblock header. This is always located as the start of an xblock.
The block trailer dialog includes an interpretation of the fields of the block trailer structure. They include:
Below the block trailer is the contents of the xblock shown as a list of BIDs of the blocks this xblock points to. Double clicking on these blocks will open the block inspector for that BID.

As an exercise, try opening the same block as Binary data, a block, and an xblock all side by side. See if you can point out how the raw binary data translates into the block trailer fields, and how the body of the block interpretation translates into the list of BIDs in the xblock interpretation.