about /dev/shm?

Hello, all.

As you know, /dev/shm is 1777 and attackers used to write his backdoors at that directory.
So I have saw the partition for some time and I can't see any creation of the files.
So I changed this permission to 755, and there was no problem.

1. What's the role of the /dev/shm?
shm means shared memory?

2. Why this directory must be 1777?
Is there any problem when I change the permission to like 755?
Thanks in advance.

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