Shortcode is a markup language used by WordPress which is used to embed dynamically generated content, often from plugins, into your posts. Shortcode is denoted by being wrapped in square brackets, [ and ].

To show shortcode in a post, you need to escape the square brackets by doubling them, [[ and ]].

(The square brackets in this comment are shown as is, I think because they’re surrounded by spaces instead of actual code.)

To create an image gallery:

1. go to Pages > Add New.
2. click Add Media.
3. click Create Gallery (on the left column).
4. shift-click the images to add
5. and click the “Create a new gallery” button.
6. Add captions if you like.
7. click Insert Gallery.
   This will take you back to the Add New Page page and you’ll see inserted shortcode like this:
   [gallery type="rectangular" ids="70,77,90"]
8. click Publish.

To add an image to an existing gallery:

1. go to Media
2. click on the image you want to add. (This will open the Attachment Details page.)
3. Look at the URL and note the item number.
   The URL will look like this:
   http://blog.stephenfranklin.info/wp-admin/upload.php?item=120
4. close the Attachment Details page and go to Pages > All Pages.
5. click on your page title to open the Edit Page page.
6. insert the item number into the shortcode ids tag. Like this:
   [gallery type="rectangular" ids="120,70,77,90"]