Due process is basically the idea that the government can't just mess with your life liberty or property without following fair rules and procedures first. It's like a constitutional safety net to stop arbitrary power trips.
In the US it comes straight from the 5th and 14th Amendments. The Fifth says no one shall be deprived of life liberty or property without due process of law and the Fourteenth applies that same protection against the states too. So whether it's federal or state level they gotta play by the book.
There are two main flavors substantive and procedural. Procedural due process is about the "how" – you get notice of what's happening a chance to be heard an impartial decision maker stuff like that. Think criminal trials where you have the right to a lawyer to confront witnesses etc. Substantive due process is more about the "what" – protecting certain fundamental rights that the government can't infringe on even if they follow all the procedures like privacy rights or marriage equality things the courts have read into it over time.
It's not some super rigid checklist though. Courts decide case by case what "due process" actually requires depending on the situation. For something low stakes like a parking ticket it might just be a hearing. For something huge like losing your kids or going to prison it demands way more safeguards.
People throw the term around a lot in politics especially when talking about arrests trials or government overreach but at its core it's about fairness and preventing tyranny by making sure power isn't exercised randomly or vindictively. Without it we'd be in a much scarier place where the state could just snap its fingers and ruin someone on a whim.
Kinda wild how something so foundational gets debated so fiercely even today huh.