Automator is a tool that will, as the name suggest, allow you to
automate things on your Mac. Automator works by files known as
"workflows", it contains a library of "actions" that you want a
workflow to perform.
Such as file renaming, creating a new email message, etc.
Workflows can be saved as individual apps that can be ran to
perform their needed task.
Additional information is available on Apple's Mac 101 article
on Automator, link available in related links below.