Oobi and Uma go to the playground. Uma goes to the top of the slide, but she gets scared and doesn't want to go down. Oobi comforts her and says that they'll ride down together. The camera switches to a profile view of them sliding down and cheering.
TVOntario stopped playing reruns of Oobi on September 2, 2006, and a rerun of "Slide!" on that day was the last time they ever aired the show. (source)
A scene from this short was featured in a musical commercial for Noggin shows called "Something New to Think and to Do." The commercial played each day from mid-2002 until April 2003. This clip is the only footage of the short that's currently available online.
The visual illusion of Oobi and Uma "sliding" is a common trick in puppetry and live-action TV. The camera angle makes it look like Oobi and Uma are on the slide, but in reality, the puppeteers are quickly moving their hands behind the slide prop. The slide obscures the rest of their arms.
The later episode "Uma Swing!" also has a scene with a slide, but the playground is a completely different set from the one shown here.