When you import a drawing into Revit, you are importing all the elements, such as blocks and external references (xrefs) from the drawing. (See .) They are all contained inside a Revit element called an import symbol.
You can explode (disassemble) the import symbol into its next highest level elements: nested import symbols. This is a partial explode. A partial explode of an import symbol yields more import symbols, which, in turn, can be exploded into either elements or other import symbols. This is analogous to exploding in AutoCAD with nested xrefs and blocks. For example, you explode an xref into other xrefs and blocks. Those xrefs and blocks can, in turn, be exploded into more blocks and xrefs.
One user's perspective:
Things you should know: Exploding an Imported CAD file (as you will read) will turn it in to either highest level elements, or lowest level ements, from CAD. (Lines, hatches = filled regions, etc). What you should know beforehand, is that doing so also litters your Revit Model with ancillary "system settings" from CAD: Every Hatch pattern becomes a Fill Pattern, all of the Line Styles from CAD get dumped in to the Revit Line Styles, all of the layers remain in the Objects Styles, and you ewnd up with hundreds- or thousands- of new "elements" because of all of the exploded CAD information. Side effects include performance degradation, instability, graphics card errors and crashing, and general all around regret.