If it's a single light
Start with the simplest explanation: a loose bulb. Turn off the switch and gently tighten the bulb in its socket. If it's an LED bulb, incompatibility with an older dimmer switch is also a very common cause — not every dimmer is rated for LED loads.
If it's one whole room or circuit
This usually points to a loose wire connection somewhere on that circuit — at the switch, an outlet, or the fixture itself. Loose connections generate heat, which makes this worth addressing rather than living with, even though it might seem minor.
If it's the whole house
Flickering across the entire home, especially when large appliances (AC, dryer, well pump) kick on, often points to a loose connection at the main electrical panel or even at the utility connection outside. This is squarely an electrician's job, not a DIY fix.
If it happens outside too, or neighbors mention it
This can indicate a problem with the utility's service to your home or neighborhood, rather than your home's internal wiring. Your utility company can check this at no cost in most cases.
Describe what you're seeing to Queen
Get a sense of urgency and likely cause before you decide who to call.