They probably mean network effects. Facebook is the prime example. For Uber, a critical mass in users and drivers makes it hard to unseat.
Each driver increases value for riders, and each rider increases value for drivers. This two sides.
A 1 sided network is facebook. A two sided market is like the video game industry (more video games are good for gamers and more gamers are good for video game devs)
For taxi apps, imagine if there was only 1 driver in SF. That wouldn't be a good experience for consumers. Thus, network effect.
Almost every user in Facebook is both a provider and consumer of content. It's like an exponentially-sided network.
In Uber, drivers provide supply to riders. Riders provide demand for drivers.
That is the definition of a 1 sided market.
Sorry, I am using economics jargon.
More drivers => decreased wait times => more customers => more drivers
Consider eBay, perhaps the most-successful two-sided network of the first dot-com boom. You could switch to another auction site, but as a seller you would lose all of your online reputation (sales history) which is worth a lot on eBay (since people don't buy expensive items from someone with no sales history whatsoever). When is the last time you cared how many drives your Uber/Lyft driver had given? eBay - very sticky. Uber/Lyft - not so much.