When you have lights flickering in the whole house it's usually caused by a loose wire in your panel box.
Your electric service panel could have a loose hot wire or even loose neutral wire that comes into your house.
The loose wire could also be in your electrical meter box or even a loose or corroded connection that attaches to your overhead wires coming into your meter box or even underground somewhere.
If you have overhead electrical service then the bad connection could be at the weather head or could be in the electrical meter box.
Another cause could be the breaker switch is loose or bad or the main breaker has a loose connection or corroded connection.
Or you could have an electric motor running such as your washing machine.
When my washing machine runs the lights in my entire house start flickering until the washing machine shuts off.
I thought something was wrong but I had an electrician come out and check things and he said it was normal for some washing machine motors to cause the flickering in the house.
It's because of the magnetic field in the electric motor.
But if you're not washing clothes or do not have any electric motors on then have it checked out because the flickering lights could be dangerous.