Okay, so imagine a calendar like we use at school or at home. It has one big square for each day, right? Well, a Julian date is kind of like that, but instead of a calendar with months and weeks, it just counts how many days have passed since a really, really long time ago.
So let's say you wanted to know what the Julian date is today. You would start counting days from this specific date that everyone agreed on a long time ago. That date is January 1, 4713 BCE (which was before calendars as we know them were even invented!).
Today is October 7, 2021. So to figure out the Julian date, we just count how many days have passed between January 1, 4713 BCE and today. It turns out that there have been about 2,458,037 days since that date (don't worry, you don't have to count them all yourself!). So that's our Julian date for today.
People still use Julian dates to keep track of things like astronomy, military operations, and scientific experiments where they need to record exactly when things happen. Because it's just a simple counting system, it's easy to use and compare across different countries and calendars.