A milestone is like a special point in time that we use to measure progress. Just like when you are building a Lego set, you might have certain parts you need to put together before you can move on to the next step. These parts are like milestones because they show that you are making progress on your Lego set.
In life, we have milestones too! For example, when you start walking, that's a milestone. When you turn one year old, that's a milestone. When you start school, that's a milestone. These special events show that you are growing up and making progress in your life.
Milestones are important because they help us see how far we've come and set goals for where we want to go next. Sometimes we celebrate milestones with a party or special treat, to celebrate our hard work and accomplishments.