First we need to qualify that Libra has nothing to do with crypto currency or blockchain and its more of a TOKEN based coin. According to Facebook they have blockchain, but the blockchain will be hosted only on there servers so it makes it centralised (XRP / XLM) coin.
Libra is more competetion to PayPal, then to any crypto coin.
In order to use Libra you need to verify your ID (they comply with all KYC and AML). After that you need to download the wallet Calibra (Facebook are working on integrating the wallet inside WhatsApp and Facebook meseenger).
The idea behind it is pretty simple. If Libra becomes major player (shops start using it, people start buying online using it etc) and Facebook integrates it inside there system, they will make alot of money by selling advertisment using Libra.
Here is a simple explanation how Libra works and why its centralised blockchain.
Instead of having a decentralised blockchain (like Bitcoin) Libra is run by Libra Association (with 28 members at the moment, hoping to grow to more then 100 by the launch time in the beginning of 2020).
At the moment financial companies in Libra are Visa and Mastercard, merchants еBay and Lyft, venture capitals Union Square Ventures and Andreessen Horowitz and none profit Kiva. All of the 28 members paid more then $10 million in order to be able to vote on the Libra and decide what happens to the TOKEN. They are suppose to check and make sure all Libra transaction are real and verified.
PayPal left the Libra Association on 4 October 2019. eBay, Mastercard, Stripe, Visa and Mercado Pago followed on 11 October, and Booking Holdings on 14 October. Vodafone joined the association in October 2019, but left in January 2020, saying they preferred to work on their mobile banking subsidiary M-Pesa.
At the moment this companys are still with Libra
• Payments: PayU
• Technology and marketplaces: Facebook’s subsidiary Calibra, Farfetch, Lyft, Spotify, Uber, Shopify 
• Telecommunications: Iliad SA, Vodafone (since left)
• Blockchain: Anchorage, Bison Trails, Coinbase, Xapo
• Venture capital: Andreessen Horowitz, Breakthrough Initiatives, Ribbit Capital, Thrive Capital, Union Square Ventures
• Nonprofit and multilateral organizations, and academic institutions: Creative Destruction Lab, Kiva, Mercy Corps, Women’s World Banking
You can check there whitepaper for more information