Multiple system calls, or syscalls, happen pretty much every time you run any command on a Linux machine. Each syscall takes a command from a human user and passes it to the Linux kernel. This tells the Linux kernel that it needs to perform some sort of privileged action. Opening or closing files, writing to files, or changing file permissions or ownership are just a few of the actions that require making some sort of a syscall. There are approximately 330 syscalls built into the Linux kernel. I can't say exactly how many, because new syscalls get added from time to time. Apart from this, syscalls differ between the various CPU architectures. So, an ARM CPU won't have exactly the same set of syscalls as an x86_64 CPU. The best way to see the list of syscalls that are available on your machine is to use the following command:

man syscalls

To get an idea of how this works, here's the strace command, which shows the syscalls that get made by a simple ls command:

[donnie@localhost ~]$ strace -c -f -S name ls 2>&1 1>/dev/null | tail -n +3 | head -n -2 | awk '{print $(NF)}'
access
arch_prctl
brk
close
execve
. . .
. . .
set_robust_list
set_tid_address
statfs
write
[donnie@localhost ~]$

In all, 22 syscalls are made from just doing ls. (Due to formatting restrictions, I can't show all of them here.)

Secure Computing (SECCOMP), originally created for the Google Chrome web browser, allows you to either enable just a certain subset of syscalls that you want for a process to use or disable certain syscalls that you want to prevent a process from using. Unless you're a software developer or a Docker container developer, you probably won't be working with this directly all that much. However, this is yet another building block for the technologies that are used daily by normal humans.

Next, let's put all this cool stuff into perspective by looking at how it's used in real life.