By default, all monitors attached to a Mac form part of a single extended desktop, if that's what you mean. In Display Preferences, you can arrange the monitors as you like, and select which is the 'primary' monitor by dragging the menubar icon to it.
There is a second layer to this called Spaces, which are multiple, virtual desktops. I.e. you can have one Space that's for video editing, and another Space for your Office apps, and then swap out all those windows automagically when you switch Spaces.
You can also have different Spaces per-monitor (that's what the 'Displays have separate spaces' option is). If you're a power user that needs to switch between apps that have multiple windows, or setups for different tasks, it's quite useful.
If you just want to have a single extended desktop, you've got that automatically, so no need to worry! Just plug in your monitors and arrange them in Display Preferences to match the physical layout.
The one quirk with this is wallpapers; you can't span a single one across multiple displays, each one has its own. (There are third-party apps to do this). I believe each Space in a display can have a different wallpaper, though, which can help you figure out which is which.