Well, IACSR stands for Individual Augmentee Combat Support Readiness. It might sound complicated, but it just means that some people in the military get special training to support combat missions, even if they're not part of a regular combat unit.
Think of it like your teacher sending some students to help out at a big school event, like a talent show. Even if these students aren't part of the talent show themselves, they can still help make it a success by working behind the scenes - setting up chairs, running the sound system, or helping the performers get ready.
In the same way, people in the military who are IACSR-trained can help out with combat missions in lots of different ways. They might work in logistics, making sure that supplies get where they need to go. They might provide medical help to injured soldiers. They might even be able to speak different languages and help communicate with locals in a foreign country.
So, even though these people might not be in a combat unit themselves, they can still support the mission in important ways. And by getting extra training, they can be really good at their jobs and help keep everyone safe.