Okay, so there's a big fight happening in Syria between some groups of people who don't like each other. They're using weapons to try and win the fight and it's been going on for a really long time.
Now, because this fight is happening in Syria, it's mostly affecting the people who live there. But sometimes, when things get really bad, the fight can spill over into other places nearby.
For example, imagine if you and your friend were fighting over a toy and you started throwing things at each other. If you accidentally threw something that hit somebody else in the room, that would be a spillover.
So, when the fight in Syria spills over, it means that the problems happening in Syria start to cause problems in other countries around it.
This can happen in a few different ways. For one, when people are fighting in Syria, they might try to run away to other places to be safe. This can cause a lot of problems for the countries they're running to because they might not have enough resources to take care of so many extra people.
Another way the spillover can happen is if the fighting starts to creep over the border into other countries. This can be really scary for the people who live there because they don't want to get hurt.
Finally, the spillover can also happen because the people fighting in Syria might start to get help from other countries. This can make the fight a lot bigger and more dangerous because more people are involved.
So, in summary, the spillover of the Syrian civil war means that the problems happening in Syria start to cause problems in other countries around it. This can happen when people try to run away, when the fighting spills over into other countries, or when other countries get involved in the fight.