Literally "to stand by things decided." It is a doctrine (not a law itself) which means you shouldn't be overturning prior judge's decisions, you should be deferential to them.
In our court system, in fact, lower judges are BOUND to stand by the decisions of higher judges. So when the US Supreme Court decides something, all lower court federal (as opposed to state) judges are bound by that decision (and sometimes even the state ones, depending on whether the issue is one that deals with a federal law's state impact, ie, like, the Bill of Rights, a State cannot overrule that).
It's an issue when discussing a matter that may come before a court that has been addressed similarly in the past. Take Roe v. Wade. It's a precedent. A Democrat would say "Stare Decisis," should be honored. A Republican would say "Roe v Wade is bad law and should be overturned by the only people who can overturn it -- new Supreme Court Justices who are not bound by the decision of the old Supreme Court Justices."
My point is that Biden is saying "Stare Decisis!" there, but Democrats are generally the ones who want to change all the old decisions.
To be fair, I have my fair share of prior rulings I'd like to see overturned (Wickard v. Filburn, Gonzalez v. Raich, the Sebelius decision (the ACA upheld under the tax clause of the Constitution), etc).