Stakers deposit tokens into stake vaults, and receive ibTOK (interest-bearing TOK) that represents their initial deposit + share of yield accumulated. The underlying TOK is matched with a corresponding token from farmers on the platform, in order to provide liquidity into an AMM. Users of the platform must ensure that farmers are within certain liquidation bounds. In short, stakers of TOK will receive back a greater number of TOK tokens than initially deposited, but the "value" (aka delta) of TOK may have changed.
Users should stake tokens to a vault if they are bullish/long on those tokens, meaning they believe the price of the tokens will not go down. Stakers can withdraw so long as the stake pool is not being fully utilized by farmers.
Interest is not guaranteed, and it can be highly variable depending on utilization and the dynamics of the vault. In the case of interest earned, the staker will receive more tokens back in-kind than deposited, but the price of these tokens may have gone down, resulting in net value losses.