LOL.
It's funny how both sides see it. Heres the narrative from the other side:
Coronavirus hits, hundreds of thousands of people will soon be dead. Some people opt to do things as remotely as possible without placing themselves at risk of dying. Others dont care. Typically Democrats are of the camp that social distancing is needed, while Trumpers keep calling it a hoax. Naturally, even early on it was evident that Democrats would vote more by mail and less in person.
Trump refuses to give the post office more funding to ensure ballots arrive on time. He knows doing so would invalidate a large proportion of mail in votes. He also knows that the majority of mail in votes will be Democratic.
Mail in voting kicks off, and an unprecedented number of votes are taken this way.
Election day comes, a lot of the votes at the polling stations have come through showing Trump to be in the lead, or about to take the majority. He wants to call it now, knowing that mail in votes are still being tabulated. Remember, those votes were cast legally and are of your fellow countrymen who's votes deserve to be counted.
Trump knew this was going to happen, and did everything he could to persuade his fervent supporters that the election was fraudulent and rigged, and they have taken the bait hook, line and sinker.
TLDR: mail in votes are legal, a lot of them were for Biden, Trump knew this and is actively trying to take votes away from legitimate voters, Trump supporters mad.