HBARX is a Hedera Token Service (HTS) token that represents your share of the total HBAR pool deposited with Stader. As soon as you deposit HBAR on the Stader smart contract, you receive newly minted HBARX, based on the exchange rate at the time of staking. As the HBAR rewards get added the value of HBARX increases (w.r.t HBAR).
To illustrate the above explanation, here is a video