Baltic air policing means that some countries in Europe (specifically, Estonia, Latvia, and Lithuania) have asked some other countries (like Germany or Italy) to help keep their airspace safe.
Think about it like this: when you're playing with toys, you might ask your friend to help you protect your toys from someone who might try to take them. Similarly, these countries have asked their friends to help protect their airspace from any bad people who might try to fly planes there without permission.
These friendly countries will send their planes to fly over the Baltic countries and watch for any planes that might be trying to cause harm. If they see anything suspicious, they will tell the Baltic countries and take action to stop them.
This helps keep the people in these countries safe, just like how your friend helps you keep your toys safe.