/etc/security/limits.conf allows setting resource limits for users logged in via PAM. This is a useful way of preventing, for example, fork-bombs from using up all system resources.
Note: The file does not affect system services. For systemd services the files /etc/systemd/system.conf, /etc/systemd/user.conf, and /etc/systemd/system/unit.d/override.conf control the limit. See the systemd-system.conf(5) man page for details.