Okay, so an attack transport is like a big boat that takes soldiers and their equipment to places where they need to fight. It's kind of like a floating taxi that can bring the soldiers right up to the beach or wherever they need to go.
The attack transport has a big open area where the soldiers can sit or stand, and there's usually a bunch of big doors that can be opened to let them out quickly. It's also got a bunch of big guns to help protect the soldiers while they're on their way, in case bad guys try to attack them.
The people who drive and take care of the boat are called sailors. They make sure the boat is going the right way and is safe for the soldiers. They also help the soldiers load and unload their equipment.
When the attack transport gets to where it's going, the soldiers can jump out and start fighting. They can use the weapons and equipment they brought with them to try to win the battles and make things safe for other people who live in that area.
That's pretty much it - an attack transport is a big boat that takes soldiers to fight bad guys in different places, and it helps them get there safely and ready to win the battles.