What is progressive overload and why does everyone keep saying it?

It means your training has to get harder over time or your body has no reason to change. The mechanism is simple: adaptation is a response to a demand it cannot currently meet, so repeating the same workout indefinitely maintains rather than builds. Overload does not only mean more weight — more reps, more sets, better technique, shorter rest, or slower controlled reps all count. This is why people who do the same routine for two years look the same. Keeping a written log is what makes it possible, because memory is unreliable about what you lifted last week.