Transient is the library used to implement the keyboard-driven menus in Magit. It is distributed as a separate package, so that it can be used to implement similar menus in other packages.
This manual can be bit hard to digest when getting started. A useful resource to get over that hurdle is Psionic K’s interactive tutorial, available at https://github.com/positron-solutions/transient-showcase.
Complexity tends to grow with time. How do you manage the complexity of commands? Consider the humble shell command ‘ls’. It now has over fifty command line options. Some of these are boolean flags (‘ls -l’). Some take arguments (‘ls --sort=s’). Some have no effect unless paired with other flags (‘ls -lh’). Some are mutually exclusive. Some shell commands even have so many options that they introduce subcommands (‘git branch’, ‘git commit’), each with their own rich set of options (‘git branch -f’).
What about Emacs commands used interactively? How do these handle options? One solution is to make many versions of the same command, so you don’t need to! Consider: ‘delete-other-windows’ vs. ‘delete-other-windows-vertically’ (among many similar examples).
Some Emacs commands will simply prompt you for the next "argument" (‘M-x switch-to-buffer’). Another common solution is to use prefix arguments which usually start with ‘C-u’. Sometimes these are sensibly numerical in nature (‘C-u 4 M-x forward-paragraph’ to move forward 4 paragraphs). But sometimes they function instead as boolean "switches" (‘C-u C-SPACE’ to jump to the last mark instead of just setting it, ‘C-u C-u C-SPACE’ to unconditionally set the mark). Since there aren’t many standards for the use of prefix options, you have to read the command’s documentation to find out what the possibilities are.
But when an Emacs command grows to have a truly large set of options and arguments, with dependencies between them, lots of option values, etc., these simple approaches just don’t scale. Transient is designed to solve this issue. Think of it as the humble prefix argument ‘C-u’, raised to the power of 10. Like ‘C-u’, it is key driven. Like the shell, it supports boolean "flag" options, options that take arguments, and even "sub-commands", with their own options. But instead of searching through a man page or command documentation, well-designed transients guide their users to the relevant set of options (and even their possible values!) directly, taking into account any important pre-existing Emacs settings. And while for shell commands like ‘ls’, there is only one way to "execute" (hit ‘Return’!), transients can "execute" using multiple different keys tied to one of many self-documenting actions (imagine having 5 different colored return keys on your keyboard!). Transients make navigating and setting large, complex groups of command options and arguments easy. Fun even. Once you’ve tried it, it’s hard to go back to the ‘C-u what can I do here again?’ way.