All the ways I can explain this are already in our documentation which I wrote myself. You are asking me to repeat myself which would either be me copying and pasting stuff from it, or saying the same things in the same way (because there's only so many ways I can explain something), neither of which would help you.
Scheduled Tasks are train tickets to go from place A to B on date and time X. If there is no train, the ticket is useless. There are three trains. The slow-ass steam engine which runs whenever someone bothers to (lazy scheduling), the run-down diesel locomotive which is great for short distances but may break down for long travel (web CRON), and the electric bullet train (CLI CRON). The passengers will get on any of the trains, whichever comes first, except the snooty passenger who only travels by bullet train (these are the "CLI Only" scheduled task types). Most tickets are for direct travel (single request), some tickets have layovers (multiple requests, like the backup task). Most tickets are season passes (repeatable tasks on a schedule), some tickets are one-way only (run once tasks). I am the station master. I have to give the right ticket to the right passenger, and make sure there are trains scheduled to come pick them up.
Try to find someone else to help you set up Scheduled Tasks. Maybe the can explain it a bit differently, in a way that "clicks" with you. I would recommend starting with something simple, like one of the sample tasks included with Joomla. The goal is not to do useful work, but to understand how it works. Something runs periodically in some way or another, goes through the scheduled tasks you have set up, and executes them based on a set of criteria. Unfortunately, I can only tell you that, not understand it for you :)
Once you get your head around how Scheduled Tasks work, their operating principle, then it's time to go to a more complex and long running task like taking backups.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!