CSRSS is a standard Microsoft service: https://en.wikipedia.org/wiki/Client/Server_Runtime_Subsystem
It is basically a intermediary function that goes between the userspace and kernelspace.
Having kernel level authority you cannot access it's memory map from a normal userspace program. This is a safety mechanism to prevent malicious programs from using the memory map of programs with access to kernelspace, such as csrss as a means to scan kernelspace memory looking for ways to gain authority escalation.
There is a widespread hoax that it is a virus or Trojan . This hoax is utilized by many unscrupulous sites who try to get you to download Trojans, Spyware or Adware in the attempt to remove it. NEVER download system scanners, or any executable files for that matter, from an untrusted website.