I cannot quite understand the quoted metaphor in the following context:
"The plates make up Earth's outer shell, called the lithosphere. (This includes the crust and uppermost part of the mantle.) Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide".
Think of the luggage carousel at an airport: where one 'plate' meets another, there's constant smooth movement. If it was 'in a state of disrepair', the movement wouldn't be so smooth; it'd be jerky.
The tectonic plates are a bit like that; the 'jerks' - where one tries to move against the movement of another - are earthquakes, tremors, and the formation of mountain ranges like the Himalayas - Wikipedia, the free encyclopedia .
PS - I took that image a bit far. Forming mountain ranges (Orogeny - Wikipedia, the free encyclopedia) would only be 'a bit like' a luggage carousel if something went dramatic wrong with it and the 'plates' were squashed together (with one sliding under another).