You would use an external flash in a low light setting to create your own controllable light source.

When using external flash, you don’t need to meter light because it automatically does it.

Flash synchronization means synching the speed to the right one. If it is out of sync, half the screen will be black and the other half will be white.

ETTL is “Through The Lens Metering”. It is the metering mode used for an external flash.

If you shoot faster than the camera sync speed, half the screen will be too dark and the other half will be too bright.

Photographers use a diffuser and bounce flash in order to bounce light from a certain direction. This is important as it is used to create lighting from an angle.

A slave is a flash mounted on the camera which creates lighting. It is what flashes the light from the camera.

If your image is too dark or too bright, you can change the intensity of the flash or point the slave flash in a different direction.