How do pull up and pull down resistors remove electrical noise?

I’m doing a project with my Arduino, and I’ve seen some tutorials on YouTube explaining how to use pull up or pull down resistors to make the input values on the board reliable, basically either a 0 or a 1 without any random changes from other em sources. The only thing is I still don’t understand why it works. The videos explain how to create the circuit but not so much the theory behind it. Any help is appreciated.

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