On a Korg synthesizer, an insert FX refers to an effect that can be applied to a specific sound or instrument within a mix, rather than being applied to the overall mix.

For example, if you have a lead sound in a song that needs a bit of distortion, you can use the insert FX to add distortion to just that sound without affecting the rest of the mix. Similarly, if you have a pad sound that needs some reverb, you can apply the reverb effect to that specific sound using the insert FX.

The insert FX on a Korg synthesizer can be accessed through the instrument's user interface or software editor. They may include effects such as distortion, chorus, delay, and reverb, among others.