From Homestar Runner Wiki
... every good road trip needs a good inside joke that only the people that went on the road trip will get. And your friends that didn't go on the road trip will want to...kill your...legs...every time you bring it up.
Strong Bad, road trip
An inside joke is something that is only found on the site in two or three places, and is obscure in most or all instances. Something that has been featured more than three times usually becomes a regular feature of the site. For instance: Trogdor, fhqwhgads, and even the main character Homsar are no longer inside jokes, even though they started out that way.
These are the things that people who are new to the Homestar Runner body of work would simply not get at all. In fact, some of those who have seen most of the cartoons may not have fully picked up on them.
The inside jokes usually come from Strong Bad Emails.