Here's a quick explainer video to get you started, scroll down for more info
Like Lightning, Liquid was purely conceptual back in 2013/14 - or rather, sidechains, which Liquid is the first official one, were purely conceptual.
The idea of a sidechain is that you can take some bitcoin, lock it on to a chain pegged to the main blockchain and then create new rules for its use, allowing the mainchain to remain clutter free and secure, expanding the use cases for Bitcoin with out compromising it.
However, Liquid doesn't quite fit the early ideals, so far. It is a federated chain, meaning you must have the permission of federated users to "peg out", which simply means to transfer your bitcoin back to the main bitcoin blockchain. There are still some remaining technical challenges to pure sidechains, but in the mean time, Blockstreams creation allows for a usable workaround to those limitations.
So what can you do on Liquid that you can't do on the main chain? Well, for a start, confidential transactions. The blocktimes are also quicker, at only 1 minute each. Then there are the tokens. Liquid provides the ability to create issue and burn tokens a lot like coloured coins did on the main chain back in 2013.
So far people are still experimenting with those possibilities and various businesses and individuals are building tools to leverage these abilities.
For your PC or Mobile
This is Blockstreams mobile wallet implementation and is the best available option at this time.
Sideswap is a service, also available as a mobile app, which allows you to peg Bitcoin to the Liquid Network without running a full Liquid server.
Specter is another provider, with a great product, who have implemented support for Liquid payments and expands on the range of options available to consumers and developers alike.