Does your kitchen lights get brighter when you turn on the garbage disposal? When this happens, it may indicate the neutral wire in the panel is loose. This means that one hot wire is worth more volts than the other, creating an imbalance according to the ground.
If your lights get brighter when you turn something else on, it can be an extremely hazardous situation. It can lead to damage to your electronic equipment, cause a fire or even electrocution. It can be even more harmful if you notice that the lights get brighter for longer than a second.
Call your local Mister Sparky electrician to regulate the situation safely.