TTL means “through the lens.” In photography, it usually refers to the camera metering light through the lens rather than with an external sensor.

Most often, TTL is used when talking about flash. A TTL flash system uses the camera’s metering to help set flash output automatically. Typically, the flash fires a quick pre-flash, the camera measures the light returning through the lens, and then it calculates how much flash power is needed for the actual exposure.

This can be very useful when you’re moving quickly, your subject distance changes, or you don’t have time to manually adjust flash power. TTL can also be used in some multi-flash setups to help balance lighting ratios automatically.

That said, TTL doesn’t automatically make photos “better” on its own—it mainly helps you get a workable exposure more quickly. You can usually fine-tune the result with exposure compensation or flash exposure compensation.

TTL can also refer more generally to normal in-camera light metering, especially on older cameras, but today it’s most commonly mentioned in relation to flash.