Redemption can refer to receiving something after a waiting period, at maturity, on selling or in lieu of a coupon.
Different financial instruments have different context for redemption.
When money market products reach maturity, investors get redemption, ie. they submit the instrument (bonds, CDs) to receive the principal sum they had invested to receive those securities.
Shareholders can get redemption (or redeem their assets) by selling shares they hold, ie. get the market price on the shares at the time of selling.
In an ETF, when authorised participants or market makers take back the underlying securities that are part of the portfolio (and mirror an index) and return ETF creation units to the ETF creator (the fund house), the process is called redemption.