Staking is the process that converts the SNX token into pooled collateral for the network. When an investor stakes their tokens, they're
minting sUSD to be used to trade for synthetic assets, whether by the staker or another investor. The staker can trade the sUSD for a synthetic asset on Synthetix Exchange or exchange it for ETH or another ERC20 token on a variety of exchanges.
Stakers earn weekly rewards for collateralizing the network. Without staking, an SNX investor can only profit from an increase in the price of the token. As of February 2020, about 80% of SNX tokens are staked. Synthetix Dashboard has up-to-date stats on the network, including the percentage of staked tokens.
Risk of staking
The rewards stakers earn are absolutely not risk-free. The staker is providing collateral for traders to trade against. If traders are profitable, net of fees, stakers will lose out in the long term. In the early days, protocol incentives de-risk the staker's role to some extent, but this calculus can change at any point.
Who can stake?
Anyone who holds the SNX token. That said, if a staker holds less than 250-500 SNX, gas costs may exceed the benefit of staking rewards.