Box is (basically) just the way to have memory on the heap. Here's a direct comparison of how to do heap memory in C/++ and in rust:

int* intOnHeap = (int*)malloc(sizeof(int));
*intOnHeap = 0;
MyClass* classOnHeap = new MyClass();

let intOnHeap: Box = Box::new(0);
let structOnHeap: Box = Box::new(MyStruct::default());

There can be a bit more to it with custom allocators etc. but that's the only way most people will use boxes. So Box basically just means "a T is allocated somewhere and we need to free that memory when this value is dropped".