Dimming lights is not unusual. Generally it is not a serious problem, but more of a small annoyance.
(i.e. A/C goes on)
When the Air Conditioner, vacuums or other appliances, that draws a lot of power, is first turned on there may be a dimming or flickering of the lights. Often resulting in a higher than normal voltage draw or fluctuation; causing the lights to dim a little. Lights that dim occurs more often in rural areas and older homes.
At times electricians may add another circuit
Occasionally there may a grounding or load balance issue in a home that may cause this. Consulting with an electrician may shed light on this.
Another cause may be that a power company’s transformer may have blown in the neighborhood. This may happen during a storm, but sometimes it just happens.
Brown out from power companies occurs at times when they experience heavy load demand.
Who to consult? An electrician.
The majority of the time an electrician can determine the cause and fix it easily. Occasionally more investigation as to causes may be required.