MRE is a tool that lets you open PRF - Portable Report Format - files. PRFs are generated by Monarch as an export from the report window.
PRFs include the report (zipped, so PRFs are more compact than the original report), as well as the tree (you can define up to five(?) fields to build this, allowing easy navigation. Think of Classic.prn, and imagine having Account #, Order #, Ship Date as tree fields. Since v9, you can now export PDFs of reports with the bookmarks in Acrobat serving as tree entries. This is limited to about 64,000 entries, so PRF files still have a place.
MRE cost about 1/10th of Monarch, so was useful when needing to send results to wider audiences. These days, if you have that kind of requirement, Monarch BI Server would give you report, table and summary views of data in a browser window - unless you can't get hold of a server and need to use some desktop alternative, I'd look at that.