ELI5 Wi-Fi - how does wifi hacking work?

Wifi explained. How does wifi hacking actually work? Can a hacker reconstruct one's screen? When the Wifi is sluggish, why does restarting router magically fix it?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: mustbefate, splad, mantice, kocholuna, AuroraAri, wace, -voiz & finalrun

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