A slight flicker is fairly normal when a big motor load such as an air conditioner or washing machine starts. If all the lights dim, then return to normal when a small appliance is turned on, then it is a system problem & the cause will be somewhere between the breakers in the service panel & the utility pole. If your neighbors are experiencing the same problem when everyone is running their air conditioners the problem could be that the the distribution grid, the transformer or the drop isn’t able to supply enough energy. Another possibility would be a loose connection at the main breaker. A qualified electrician will be able to find & fix your problem, or rule your side out & get your service provider to inspect it’s side. If the problem is in the main panel, someday it will fail catastrophically, so that situation would be urgent.

It’s a different problem if some lights dim & some get brighter, this would indicate a compromised neutral connection between the service panel & the transformer. This is an immediate danger because if the neutral connection is lost higher than normal voltages will destroy or damage appliances & could start a fire. The utility service provider would normally respond immediately if this condition is found.