So, when we're born, we can't do much. We can't talk, walk or even hold our heads up by ourselves! But as we grow, we learn how to do all these things and more - we call these things "developmental milestones".
For example, when you're a baby, you learn to roll over, sit up, crawl, and walk. When you're a little older, you learn to talk, and then you start to learn things like counting and reading.
These milestones are things that we expect most kids to be able to do at certain ages. For example, we expect most babies to be able to sit up by themselves by around 6 months old, and to start walking by around 12 months old.
Different kids might reach these milestones at slightly different times - some might learn to walk a bit earlier, for example - but if a kid is much later than usual in reaching a milestone, that can be a sign that they might need extra help or attention from grown-ups.
So overall, milestones are just things that kids learn how to do as they grow up, and they help us see how kids are developing and learning over time.