What is Obstruction of Congress?

I'm not sure that I fully understood the term Obstruction of Congress until today.  I also didn't really understand what executive privilege was in relation to the impeachment proceedings until today.  The House chose not to subpoena certain people because they knew it would delay the impeachment proceedings for months, if not longer.  How did they know this?  Because the White House forbade people from testifying or handing over documents.  Under what grounds did the White House do this?  They didn't do it for purposes of executive privilege, but rather to avoid having any evidence come forth that would be relevant to the impeachment proceedings.

Executive privilege makes sense when the topic is sensitive to national security.  That means that portions of testimony could very well be found to be protected by executive privilege, but certainly not everything.  However, a blanket refusal to share any information is what the White House did.  I've striven to be objective, but I do not understand how telling everyone that works under you not to testify doesn't represent Obstruction of Congress.