Working with code blocks is one of the great features of Org mode and is therefore rather well known.

They canonically look like this:

#+NAME: <name>
#+BEGIN_SRC <language> <switches> <header arguments>

This works great when you want to add either a bigger snippet for an exported document or you want to do actual calculations with data from the document.

For the latter case, if you don't need a whole lot of room for your code - for example if you're doing an easy calculation like the VAT on an invoice, it would be great if you had the ability to add inline code blocks. And of course, Org mode provides!

Anywhere in an Org mode document, inline code snippets can be embedded like this: src_elisp{( + 1 2)}

When you hit C-c C-c while point is on this code, it will execute and append the result. When exporting, the code block is not visible while the result is.