What It Means
Theta is an option Greek that measures how much an option's premium is expected to decay each day purely from the passage of time, all else being equal — usually expressed as a negative number, since time decay works against an option buyer.
How It Works
Every option loses some time value every single day it doesn't expire, simply because there's one less day left for the underlying to move favorably — this is true whether the underlying moves or not. Theta isn't constant: it accelerates sharply in an option's final one to two weeks before expiry, which is why the same option can lose noticeably more value in its last few days than it did over the preceding month. Option sellers benefit from theta (time decay works in their favor as the seller collects premium), while option buyers are fighting it — the underlying has to move enough, and fast enough, to outpace the daily erosion in value.