xMARS tokens can be obtained by staking MARS tokens in the protocol.
When the protocol launched, each MARS token was stakeable 1:1 for xMARS. As the protocol accumulates fees from borrowers, 10% of those fees will be programmatically used to purchase MARS and add it to the xMARS pool. That means that over time, each xMARS will be redeemable for more MARS than was initially staked.
For example, at the moment of launch, if users staked 100 MARS tokens, there was 100 xMARS in circulation. Over time, the staking pool would grow to say 200 MARS tokens as the protocol accumulated fees, purchased MARS and added it to the staking pool. If someone staked an additional 100 MARS at that point, they would have received 50 xMARS. The staking pool would then hold 300 MARS, and there would be 150 xMARS will be in circulation. At that point, each xMARS would be redeemable for 2 MARS.
Note that each xMARS token is always worth 1 unit of voting power on the Martian Council.