This is true for a direct democracy, but for a representative democracy like the US (and many other countries) there's more nuance. Combined with first past the post voting, there's a lot of room for suppression of voices that are not aligned with those already in power.

For say something like the state legislature race in a state, they count up all the seats they have won in each district, and whichever party has won the most seats wins the race. Voters are therefore put in buckets (districts) and their votes are counted in aggregate.

This allows a process called "gerrymandering" to redraw district boundaries arbitrarily. So if there is say a democratic-party majority voting bloc in a particular area, I can redraw the surrounding districts to split that geographic area into many parts, so their votes get split across multiple districts and hence "diluted".

The voting rights act asserted this is a form of voter suppression. Specifically related to black voter suppression, if a state is say 40% black by population and they have no black representatives, it warrants a closer look as to why.

I hope that wasn't too confusing of an explanation. I'm not from the US but I'm quite interested in these things.