‘Meta’ is a broad term these days, but for our purposes we can consider it to be ‘something that goes beyond’, or ‘something that contains something else’. This cartoon, from XKCD.com is an example of meta, and meta humour:

There are some layers to be uncovered here, including that the subject/original reference if Douglas Hofstadter (known for his ‘meta’), the acronym itself becomes ISMETA, and that the set up in the first screen refers to a brief biography – hence giving the exercise a purpose.
It is a good joke. Clever.

Being able to step outside of something, making an object of something that was previously the subject is ‘meta’. It is above or beyond or outside the surface structure. Just as the A-Team referenced Battlestar Galactica in the opening sequence, the meta humour is found not at the level of ‘story’ and characters, but at the level of ‘actors’ i.e. revealing to the audience, in a questioning glance, that the actor themselves is in on the joke. Without knowing this extra piece of information i.e. that he acted in both TV shows, you will never get the joke. So in this way, we have a direct ‘intertextual’ reference too, hence the frequent connection between these two forms.

In a similar manner we can see ‘meta intertextual humour’ in the movie ‘The Martian’, where the scientists have a conversation as to the decision to name a project. Here is the scene…

Annie Montrose: What the hell is “Project Elrond?”

Vincent Kapoor: I had to make something up.

Annie Montrose: But “Elrond?”

Mitch Henderson: Because it’s a secret meeting.

Annie Montrose: How do you know that, and why does Elrond mean secret meeting?

Bruce Ng: The Council of Elrond. It’s the… it’s… it’s… from The Lord of the Rings. It’s the meeting where they decide to destroy the one ring.

Teddy Sanders: If we are going to call something Elrond, I would like my code name to be Glorfindel.

Annie Montrose: I hate every one of you.

Now, being this won’t make any sense to you on the surface unless you know that in The Martian, Mitch Henderson is played by Sean Bean, who also plays the character Boromir in Lord of the Rings – hence the meta intertextual humour.

For the smartest of you, you’ll probably be wondering the difference between ‘meta intertextual’ and plain old ‘intertextual’. The examples above are perfect ways to see the difference. For intertextual references, you may consider them to be a reference to another movie, book, tv show, historic event etc. When you add the meta layer, once more, it ‘goes beyond’ that referencing and you fall into a more complex layer of mind.

Intertextual is a ‘link’ on the same level; ‘Meta’ is a leap to another. When you put the both together you experience both within the same space of mind, simultaneously – and it is that that you know as the experience of mind for a meta-intertextual moment.