Little's Lemma is like counting kids in a playground! Imagine there is a playground with a fence around it. You want to know how many kids are playing inside the fence. You can't go inside the fence, but you can stand outside and count how many kids go in and how many kids come out.
Now, let's say you count for one hour and you see that five kids go in and five kids come out. Little's Lemma says that the number of kids in the playground at any given time is the same as the number of kids who went in minus the number of kids who came out. So, if five kids went in and five kids came out, there must have been five kids playing in the playground the whole time!
This is similar to how Little's Lemma works in computer science. It helps us figure out how long something will take in a computer system. Just like counting kids, we count how many jobs come into a computer system and how many jobs leave. And just like in the playground example, Little's Lemma tells us that the average number of jobs in the system is the same as the average number of jobs coming in minus the average number of jobs leaving.
So, if we know how many jobs are coming in and how fast they are being processed or leaving, Little's Lemma helps us figure out how many jobs are in the system and how long they will take to be processed!