An anti-ballistic missile is a special type of missile that is made to protect people or places from being hurt by other missiles. Imagine there is a big bad guy who wants to throw a ball at you really hard, and you have a shield to block that ball so it doesn't hit you. Well, an anti-ballistic missile is kind of like that shield, but for big missiles.
When there is a missile that might hurt someone or damage something important, the anti-ballistic missile is fired into the sky to intercept it. It is very fast, like a superhero flying through the air to catch a bad guy. When it reaches the bad missile, it uses special technology to either explode near it, or to actually hit it and knock it out of the sky.
This helps keep people safe from dangerous missiles or attacks, kind of like having a really good goalie in a soccer game. The anti-ballistic missile can't stop regular things like planes or cars, but it is important to keep us safe from bad people who might want to hurt us with big missiles.