What you're ultimately talking about here are "conditional" order types. First - you don't absolutely NEED them. They're very handy, and will make your life easier because they remove the need for "basitting" your accounts, but you can construct a perfectly viable trading strategy without them.
That said, for certain kinds of traders (esp. day traders and folks who are very active) they are pretty close to essential (but that's not how most people start out).
As far as how to use them...well, let's make up a fictional example.
Let's say you're interested in buying X. X can be whatever. A stock. A CFD, Cryptocurrency - doesn't really matter. It's an investment you're interested in and you've been watching its price bounce around.
So, since you've been watching it, you've noticed that right now, it's trading in a range between $30 and $35.
The idea is to buy low and sell high, so you don't want to buy any of X if the current price goes above $35, and you don't want to sell any if it drops below $30 (because then you'd be buying high and selling low, and nobody ever made any money doing that!).
So you set up a conditional order where you tell the system you want to buy at say $30, and sell at $35. Either or, and on some platforms, you can set that up all as one conditional order, with one cancelling the other.
So if your order triggers a buy, then it will cancel out the "sell" part. Or if you sell, then it will cancel out the buy. That gives you, the investor, a chance to review what's going on and make a new order (or conditional order) based on changes in the market...because the stock could be tanking, or it could be breaking out to trade at a new, higher price band. Either way, you'll want to plan and re-assess before you do anything else.
It's basically a handy way of covering all your bases. Since you don't know exactly what the market's going to do from hour to hour or day to day, you set up something like that so you don't have to know - you don't have to sit staring at your screen constantly with your finger on the mouse button. You can just set up the parameters and leave it, then check back in once in a while to see if those conditions still make sense, given what the market is doing, or check back in when one of your conditions triggers.
Make more sense? 🙂
(and Mods - if I got some part of that wrong, all means, feel free to correct me - I'm still learning too!)