Theta is the option price change per calendar day of time passing, negative for owned options and collected by short ones. Decay accelerates into expiration and is steepest for at-the-money strikes.
Theta and gamma are two faces of the same trade: the decay a short position collects is compensation for the gamma it owes. That is why "more theta" is never free; the shortest expirations pay the most per day and carry the most breach risk per move. The practical harvest window for credit structures, roughly 21-45 days to expiration, is where the theta-to-gamma exchange rate is most favorable, covered in expiration selection.