To 'step on a joke' is a term in comedy writing. It means to ruin a decent joke by by following it too quickly with a riff or another punchline in an attempt to draw more laughs. Stepping on a joke is a fatal flaw of bad sitcoms and improv groups, where several actors may step on each other's jokes in attempts to one-up each other.
Joke-stepping is particularly noticeable in certain comics, where attempts to cram several related punchlines into a short space such as a final panel are more obvious than in filmed media.
In some usage, to step on a joke means to ruin it by deflating it or explaining it. This is more accurately called 'killing the joke' which can be a deliberate form of anti-humor. Stepping on the joke usually isn't.
To use the phrase:
Did you see that Jay Leno monologue? Making all those lame comments after his punchline? He stepped on that joke so hard he practically curb-stomped it.
To step on the joke:
It was like he was going, "Kiss the curb, you joke!" Heh. Hmm. Yeah. Whoo.
*Applause sign flickers*