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.
Here is a sample mode line for a window containing a file named dir, showing the node ‘Top’.
-----Info: (dir)Top, 40 lines --Top-------------------------------------
^^ ^ ^^^ ^^
(file)Node #lines where
Truncation of long lines (as opposed to wrapping them to the next display line, see toggle-wrap) is indicated by a ‘$’ at the left edge of the mode line:
--$--Info: (texinfo)Top, 480 lines --Top--------------------------------
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---------------------------------