According to the tweet, members of the Cardano family can familiarize themselves with features on the testnet, in anticipation of the mainnet.
“Today marks the start of our public Shelley testnet, a hugely important milestone on our shared journey to Shelley mainnet. From right now, any operator who wants to run a Cardano pool can start getting ready for mainnet staking & delegation later this summer #ShelleyTestnets.”
According to IOHK marketing director Tim Harrison, the Shelley testnet is approaching its last phase. The mainnet is expected to go live sometime in July 2020.