At its most basic level, Theta is the rate of decay an options price experiences over time. Keep in mind Theta is not constant and the rate of decay will tend to increase as expiration nears.

Also, keep in mind that although each greek references a certain aspect of options pricing they need to all be thought of in the sense of 'all things being equal' and 'at this exact moment'. In other words as price, time or volatility etc change, so too do the greeks.

The options theta is a measurement of the option's time decay. The theta measures the rate at which options lose their value, specifically the time value, as the expiration date gets closer. Generally expressed as a negative number, the theta of an option reflects the amount by which the option's value will decrease every day. In other words, an option premium that is not intrinsic value will decline at an increasing rate as expiration nears. The Theta is one of the most important Options Greeks. Get a brief answer to your query with some live examples here. https://steadyoptions.com/articles/options-theta/