In a nutshell, tags allows you to navigate through program code distributed over multiple files effectively. e.g if you see a function call or a struct in e-code and want to "jump" to the definition (which may be in a different file) then you just hit CTRL+] in Vim! Pressing CTRL+t will take you back where you came from. Check out http://vim.wikia.com/wiki/Browsing_programs_with_tags#Using_tags if you want to learn more about how to use tags with Vim.
This utility can generate tags file for your e files. It can either walk through e import order, a directory recursively or all directories on SPECMAN_PATH recursively! The tags file will have tags for struct, unit, types, events, defines, fields, variables, etc.
For help and some examples, just run ctags4e -help.