Very specifically, they use that term so that low information and dyed-in-the-wool Dem voters will throw it back at you. It works like this (You in this example is the 'general you'):

You: "There is fraud." Them: "No. Prove it." You: "Ok, here is an example (example given, with evidence)" Them: "Well that's just one case. It doesn't prove widespread voter fraud."

Now here is the mistake most people make (me included). You either keep trying to show them case after case and they keep saying those are just isolated incidents, or you throw up your hands and get angry because you can't believe they aren't getting it.

Here's what we need to do:

Them: "Well that's just one case. It doesn't prove widespread voter fraud." You: "The reason that your side, the Dems and the media, keep saying that is because they know that no one has the time or patience to lay out every... single instance of voter fraud in the entire country. Every example I give you, you'll just say it's one case. I could show you 1,000 examples where 100 votes were flipped fraudulently and you'd say "those are just individual cases". Do you know what 1,000 times 100 is? 100,000. If Trump had 100,000 more votes in key areas, he wins in a landslide."

TL;DR - They use the term "widespread" so that you can't give an infinite number of examples in a single conversation.

Always have specific a couple solid examples to give in an argument, but never let anyone drag you into an argument of specifics. You MUST win the overarching logical points, then make them go do the work to find the specifics. If they want to disbelieve after that, they are beyond you saving.