It means that if the child process actually modifies the memory then those modifications will be visible in the parent process, because they're both using the same address space. It's essentially an awful hack that was added to BSD at a time when it didn't yet use copy-on-write for fork() to achieve the same performance with vfork()+exec() that you would get from a CreateProcess()-like API.
That's why the child is allowed to do almost nothing:
the behavior is undefined if the process created by vfork()
either modifies any data other than a variable of type pid_t used to
store the return value from vfork(), or returns from the function in
which vfork() was called, or calls any other function before success‐
fully calling _exit(2) or one of the exec(3) family of functions.
What does that mean?