CSS is an integral part of the web page which makes the content and web page appealing.
Leaning all the CSS seems unachievable but learning it to an extent where using and applying it to our website, is important. So to do that there are a few simple steps that will eventually lead us to the path which clears the "CSS Concept".
These steps can only be followed, if you are familiar with CSS a little already.
Go to a simple website that you like or use regularly. You can choose any social media platform until it's not complicated or you can also go for any website (it'll be even better if it's unpopular).
Start making a component library by replicating the links, sections and buttons or what ever that website contains.
While you are making the component library, make sure you are doing it in a form that you can use it later anywhere you want.
Now, it's definite that you'll get stuck somewhere. That's the point you want to push and learn a bit more to improve. And here comes the trick, "drum rolls please".
Look at the source code of the object you are replicating.
Right click on the object and click "inspect", there will be your code. Look at it, try understanding it and replicating it in your component library.
That's it, doing it for a few website will shape your CSS skill in a good way and make you write a better CSS for life. Practicing it this way will lead you to clear concept of CSS.
If you would like to suggest any other way, you can tweet it to me. I would love to read.