What does 'd' do in oh-my-zsh?

The d command is a function defined in directories.zsh. It lists the recently visited directories, which you can navigate to directly using the number aliases defined in the same file. So for example:

➜  ~/.oh-my-zsh/plugins (master) ✗ d
0   ~/.oh-my-zsh/plugins
1   ~/.oh-my-zsh
2   ~/.oh-my-zsh/lib
3   ~
➜  ~/.oh-my-zsh/plugins (master) ✗ 2
~/.oh-my-zsh/lib
➜  ~/.oh-my-zsh/lib (master) ✗

Looks like a history of most recent directories you've been in. which d identifies it as an alias to dirs shell builtin, which prints the contents of the directory stack. Just tried it and number keys allow to move to respective directory. Cool feature :)