These files contain ImageJ macros. You open a macro file using 
File->Open and run it using editor's File->Run Macro command (ctrl-R).
You can also open macro files by dragging and dropping them on the
ImageJ icon (Macintosh) or on the main ImageJ window (Windows). You
can run a small piece of macro code by selecting it and typing ctrl-R. 
Some of the files (e.g., MacroSet.txt) contain macro commands that
get installed in the editor's "Macros" menu.

Macros can also be installed in the Plugins>Macros submenu using
the Plugins>Macros>Install Macros command. Macros in a file named 
"StartupMacros.txt" are automatically installed in the Plugins>Macros 
submenu when ImageJ starts up. The "MacroSet.txt" file contains 
example macros that have keyboard shortcuts. Note that these shortcuts 
only work when an image window or the ImageJ window has focus.

The tools folder contains tool macros that are added to ImageJ's
tool bar when you open the macro file.

The macros.zip file is a compressed ZIP archive containing all the 
macros in this folder.

A list of the built in macro functions is available at

     http://rsb.info.nih.gov/ij/developer/macro/functions.html