It’s a terminal framework that lets you create custom configurations for your terminal through the ~/.zshrc file. You can edit it with: code ~/.zshrc
MacOS 10.15 Catalina and Above
Apple switched their default shell to zsh, so the config files include ~/.zshenv and ~/.zshrc. This is just like ~/.bashrc, but for zsh. Just edit the file and add what you need; it should be sourced every time you open a new terminal window:
nano ~/.zshenv alias py=python
Then do ctrl+x, y, then enter to save.
This file seems to be executed no matter what (login, non-login, or script), so seems better than the ~/.zshrc file.
Examples
iA Writer
open nano ~/.zshenv and add ia="open $1 -a /Applications/iA\ Writer.app" to open MD files with ia filename.md
OR alias -g ia="open $1 -a /Applications/iA\ Writer.app"