Theta is a measure of how much the price of an option will change in relation to the passage of time. Theta is often expressed as a decimal, and represents the change in the value of an option for every one-day change in the time to expiration.

For example, if an option has a theta of -0.01, that means that for every day that passes, the option's price will decrease by $0.01. This is because as time passes, there is less time for the underlying asset's price to move in a way that would make the option profitable, hence its value decrease.

Theta is particularly important for options traders because time decay is a key consideration when trading options. The closer an option is to expiration, the more rapidly its value will decrease. As a result, options traders need to be mindful of the time remaining until expiration when managing their positions.

Theta is also important for managing a time neutral position, as it measures the rate of change of time, it helps traders to adjust their positions accordingly to keep them time neutral.

Theta is not constant, it changes as the underlying price changes, volatility changes and time passes. It is highest for at-the-money options and decreases as the options goes further in or out of the money. It is also affected by the volatility of the underlying asset, high volatility options have a slower theta decay than low volatility options.

In summary, Theta is a measure of how much the price of an option will change in relation to the passage of time. It is important for options traders to understand and monitor Theta when managing their positions, as time decay is a key consideration when trading options.