A mode line is a line of inverse video which appears at the bottom of an info window. It describes the contents of the window just above it; this information includes the name of the file and node appearing in that window, the number of screen lines it takes to display the node, and the percentage of text that is above the top of the window. It can also tell you if the indirect tags table for this info file needs to be updated, and whether or not the info file was compressed when stored on disk.
Here is a sample mode line for a window containing an uncompressed file named `dir', showing the node `Top'.
-----Info: (dir)Top, 40 lines --Top-------------------------
^^ ^ ^^^ ^^
(file)Node #lines where
When a node comes from a file which is compressed on disk, this is indicated in the mode line with two small `z''s. In addition, if the info file containing the node has been split into subfiles, the name of the subfile containing the node appears in the modeline as well:
--zz-Info: (emacs)Top, 291 lines --Top-- Subfile: emacs-1.Z-
When Info makes a node internally, such that there is no corresponding info file on disk, the name of the node is surrounded by asterisks (`*'). The name itself tells you what the contents of the window are; the sample mode line below shows an internally constructed node showing possible completions:
-----Info: *Completions*, 7 lines --All---------------------