Right now, there are three ways this can go.
Best possible outcome here is that Biden wins by a significant amount, so that there can be no real question about the rightful victor. Trump might still complain, but there won't be much he'll be able to do, beyond encourage some of his supporters to protest in the streets ineffectually. Unfortunately, it's looking like this won't be the case. Given the current numbers, it's highly unlikely, though still not impossible. (Specifically, Biden would need to take Pennsylvania, Michigan, Wisconsin, Arizona, and Nevada, despite currently being behind in the first three states.)
Worst possible outcome is that the election comes down to the wire, similar to the Bush/Gore race in 2000, but even worse due to disputes about mail-in ballots and deadlines. If that happens, this whole process could drag out for days, maybe weeks. There'll be heated disputes in the legislatures and in the courts, on the state and federal levels. There's almost no chance that we'll see a literal civil war or revolution, as some people are claiming, but there will be severe civil unrest as both sides take to the streets. Imagine the Kenosha riots crossed with the 2017 Charlottesville rally, but in every major city across the nation, and you'll have an idea of what to expect. There's also no guarantee who'll ultimately end up in the White House: depending on how various politicians, lawyers, and judges vote (and what the political makeup of the new House and Senate look like), we could end up with 4 more years of Trump or Joe Biden or Nancy Pelosi or Chuck Grassley or someone else entirely. It'll basically be a spin of the roulette wheel.
Another bad outcome is simply that Trump wins, fair and square. I really, really, really don't want this to happen. It'll be absolutely disastrous for our nation, especially for marginalized groups. Trump will allow the pandemic to continue spreading, he'll crash the economy with his protectionist policies, he'll strip away welfare programs and healthcare coverage, he'll continue locking immigrants in cages, he'll overlook and encourage police brutality, he'll crack down even harder on peaceful protesters, he'll keep waging wars in the Middle East, he'll appoint even more ultra-conservative judges to push back against women's rights and LGBT rights. Still, I'd prefer a fair Trump victory (or at least as fair as we can get in a country with an undemocratic Electoral College and widespread gerrymandering), because if Trump takes the White House with some post-election fuckery, we'll get all of the above and widespread civil unrest for several months and further degradation of our already flawed and fragile democratic norms.