A simple answer: click them and find out. You will have some idea of what they do because of the labels (text, border, background etc.). In some cases, Wappler includes quite a bit of information too.
Eg there are lots of options for background images in CSS. These each have brief explanations in tootips. Here I’ve hovered over ‘Cover’ and the relevant information appears:
… but this may not very clear - not until you see it in action. With Wappler you can see all the options, then see what they actually do and also see what the code looks like. It’s a great learning tool from that point of view.
You can learn a lot from just clicking and seeing how the interface responds. You could open one of the sample templates and start clicking on objects in the Design panel; you will see the relevant components and options highlighted in the panel you have open (App Connect, Design etc.). You could then try to change some of the settings. You should probaby use the App Connect panel first - here you will see the existing options which Bootstrap offers; selecting an option here will not produce any additional CSS code because it already exists. If you use split view, you will see exactly what’s happening in the code - ie which Bootstrap classes are applied. If the styles you want aren’t there, experiment with the Design panel and try out the options there.
I think you could probably learn a lot about Wappler and CSS in this way. Having said that, having some basic understanding first is really the best way to go, and as Teodor pointed out, there are thousands of tutorials, courses and videos on the subject available. You could get quite a long way without any existing knowledge but it would be a little like the way most people use Word - they just click buttons until they achieve the results what they want. This is fine up to a point, but if you want to produce professional documents in a flexible and efficient way, then you will take a little extra time to learn about styles (and save a lot of time in the end).