Flickering covers several different problems that happen to share a symptom, and separating them is mostly a matter of noticing where and when.
If a single fixture flickers and nothing else does, start with the simplest causes. A bulb not fully seated. A worn socket contact. In dimmed circuits, a bulb that is not compatible with the dimmer, which is extremely common since LED lamps replaced incandescent ones. Older dimmers were designed for a resistive load and misbehave with the electronic drivers inside LED lamps.
If several fixtures on the same circuit flicker together, attention moves to the circuit. A loose connection at a device, at a junction, or at the breaker itself produces intermittent contact. The characteristic pattern is flickering that responds to vibration, someone walking across the floor above, or a door closing.
If lights throughout the house dim briefly when a large appliance starts, that is voltage drop, and a small amount is normal. Motors draw a large inrush current for a fraction of a second when they start. A brief, slight dip when the air conditioning or a well pump kicks in is ordinary behaviour.
What is not ordinary is a deep, sustained dim, or dimming that happens without any obvious load starting.
The pattern that deserves urgent attention is lights across the whole house flickering or changing brightness independently of anything you are doing, particularly if some circuits go bright while others go dim. That combination can indicate a failing neutral connection at the service, which allows voltage to divide unevenly between the two legs of the supply. Devices on the overvoltage side can be damaged quickly.
Also worth taking seriously is flickering accompanied by any smell, any warmth at a switch or outlet, or any buzzing from the panel.
The diagnostic sequence that costs nothing is to note whether it is one fixture, one circuit, or the whole house, and whether it correlates with anything switching on. Those two observations narrow the cause considerably before anyone opens anything.
A useful reference: electrical troubleshooting guide