Retab is a small command line utility that allows you to reformat ASCII text files so that the tab characters represent a different number of characters. It can reformat text for new tab size, convert all tabs into spaces or it can convert spaces into tabs.
I wrote this a long time ago when I had a printer which printed out tabs as 8 spaces. This meant that I couldn't use this printer with some files in which tabs represented a differented number of spaces. Retab allowed me to print these files with that printer.
If you just want to see what it does, you can read the user manual.