The light brightness is related to the amount of power it receives; more power means a brighter output.
If the power is applied for a longer duration every AC cycle, then there is more power being delivered to the light and it will consequently be brighter.
If the power is applied for a lesser duration every AC cycle, then there is less power being delivered to the light and it will consequently be less bright.
TRIACs are generally only used for switching AC loads. This includes switching heaters, lighting, motors, etc.