Custom Plugin Tag in Post

Hi everyone,

I’m still pretty new to WordPress plugin development, but I’ve been enjoying it so far. For my plugin that I’m writing, there’s a feature that I’d like to implement that I’ve seen other plugins use, but I have no clue what it’s called or how to do it.

My way to describe it is a “tag” that you are able to put in a page or post and it’ll run in the content. One of the plugins that does that functionality is NextGen and their “tag” looks like:

[nggallery id=x]

Could someone please be so kind to direct me to the correct location on whatever “that” is? Thank you.