A light can slightly dim or get brighter when your air conditioning system or other major appliances turns on or off, respectively. A slight change like this is relatively normal. In some cases, a light might flicker due to incompatibility between a dimmer switch and the type of lighting fixture. These are fairly common and are normally not cause for concern.
In contrast, where you really need to pay attention is if your lights flicker randomly, or even turn off and on intermittently. Assuming you’ve already checked to make sure the bulb or bulbs are secured correctly in their respective socket, this symptom is usually a sign of an issue somewhere within your home’s electrical system, such as a faulty or loose breaker in the main electrical panel.
In reality, it may not be lights that you notice, but maybe a TV or other device that behaves erratically, either on its own or when other loads are incurred on your electrical system (such as turning on lights in the next room).