Welcome to the group! SmartThings can be great fun for people who enjoy coding, it’s a very flexible developer platform.
From a coder viewpoint:
Mode functions as a global mutable state variable. So you can change it from a lot of different places and then it’s available everywhere. And yes, if you’re not careful it will break your heart, because one process may have changed it before another one checks it. 
(And it can be very hard to debug because an error may depend on which process happened to complete first.) Mode can only have one value at a time, but you can add more values if needed.
And you can make mode one of the schedule controllers and change the mode at the end of the action, like changing mode from At Home to Away.
Hello Home Actions are limited option scenes with limited scheduling built in. You can’t set every every parameter for every device from a standard Hello Home Action, but you can set quite a few. You can set some specific triggers, like all presence sensors outside the geofence on Monday and a motion sensor goes off, and have a bunch of different actions occur on different devices at about the same time. Like the door locks, the thermostat setting goes down, and the bedroom lights go off.
From a non coder point of view:
Hello Home Actions are a way of controlling when certain action requests will be sent to the devices in your home, and grouping multiple action requests together so they all happen at about the same time.
The “when” doesn’t just have to be based on time and date–it could be defined by a specific door being closed. Or a switch being flipped. Or when the last person leaves the house on a weekday. So when the last person leaves the house on a Monday, the door might lock, the thermostat adjust to a new setting, and the bedroom lights go off.
Mode is a very powerful way of controlling how specific actions are triggered that isn’t necessarily based on date, time, or another device doing something.
“Vacation” is a typical mode. Instead of saying how things should happen differently based on a specific date, you could set up a Vacation mode and then reuse those settings anytime you were going to be gone from the house overnight.
Different people use modes differently. Some just stick with a basic 3 or 4, like At Home, Away, and People Sleeping. Others use many, like Housecleaner, Kids After School, or Guests Visiting.
You can only have one mode set at a time. This is why some people end up with many modes, like one for Dogwalker during Vacation, one for Kids After School with Guests, etc. SmartThings gives you complete control over how and why you will use modes.