The MacOS Activity Monitor will provide you with a list of all the applications you are running, which is useful for shutting down processor intensive processes. But it also introduces a bunch of system processes, some of which may not be safe to stop smoking. Here's how to make the difference.
Who are all these users?
First of all, you should look at who owns the process. Processes in macOS (and any other Unix-like operating system, including Linux) have owners, linking each process to the user account that initiated the process. And even if you recognize your user account, there are many other users on your computer, most of which are managed by the system.
You can see here that on a standard macOS installation, there are over 250 system-managed users, most of whom begin with an underscore:
Macs have so many user accounts because of the way permissions work in macOS, and each user has specific permissions. For example, _dock would have permission to access the files related to the dock and not much else. This further secures your system by keeping low-level system processes in their own containers.
Important: Since most of these are purely system processes, it is best to never leave a process that begins with an underscore.
It is probably safe to close all processes under the name of your user account because most of them will restart automatically if they are needed. However, you should not go overboard by closing everything to save on system performance because the vast majority of processes running on your computer are inactive. It's better to leave them where they are needed instead of spending extra resources to reopen them.
Processes with an icon next to their name indicate which applications can usually close safely. You can sort by "% CPU" to display applications using the most resources:
Some of them, such as Google Chrome, will have support processes used to improve performance. You will want to exit applications such as Chrome from the Force Quit menu (Option-Command-Esc) rather than from the Activity Monitor.
One thing to note is that if the application has one of the two icons below, you should pay more attention when you close it:
Icons to watch for are a white sheet with a pencil, a brush and an "A" shaped ruler or shield.
The first is the default icon for an application without an application, which may mean that it is a background process that does not require an icon for the user. The latter is an icon specific to user-level Apple processes, such as Siri, Finder, and the Dock.
What is the "root"?
The next step is the root, which is the user account with the most system permissions. This is a strange case because most processes in the root account process system processes, but a few things you start will be started as root, especially those that need to access low-level system resources. These are more difficult to detect because you must know what you are looking for:
Here is an example: ckb-next is a third-party driver for my Corsair USB Mouse. So I know that ckb-next-daemon, running as root, is an auxiliary process for this application. If I closed it, my mouse would stop working. In general, if you see something that you recognize as being running as a root user, it may be safe to close, but most of the processes in this category are system items that you should not to touch.
In the View menu of the top menu bar, you can edit the processes to display. You can choose to display only processes with windows, which will display the same list as the Force Quit menu. You can also see the processes initiated by you, by the system, as well as those that are active or have become inactive.
The useful part of these filter views is that you can then sort by "% CPU" in addition to that. For example, you can view the longest processes in the system by choosing "System Process" as the filter and "CPU Time" as sorting.
Whatever you choose to stop, you can not really hurt your Mac, because any damage you could do can be repaired with a simple reboot. In fact, the best way to clean up the list of processes is to restart your computer, which will eliminate some unnecessary things. Look for applications that start working as soon as you log in and uninstall the ones you do not need.