In simple terms,
CSS reset is resetting the default values of CSS that are present on all elements which are defined on the DOM.
Now this naturally begs the question, what are the defaults on an arbitrary HTML element defined on the DOM?
The answer is same as almost all questions related to computer science in general... it depends
...it depends on the HTML element we take into consideration,
Let's look at some examples.
Take the h1 tag, it has a CSS default of font-weight: bold. This one is a very intuitive example and it kinda makes sense for the headings to be bold.
Let's look at a contrived example to really understand why we need to do bother doing a css reset before starting to write any css at all.
You may have noticed that whenever we add a header to a page we have a margin on the top and bottom automatically present.
If you take a look at the above StackBlitz code editor, we see that the h1 in red has some space on the top and bottom of it, it is the margin which is by default present on a h1 tag.
So, for the second and third headers we have set margin: 0 which overrides the browser default. You can see how both the headers have their margins overriden.
There are so many defaults given by the browser on several elements which makes it hard to keep track of and also these defaults tend to rather give us a lot of uncertainity in our CSS.
You can take a look of the all the defaults on HTML elements in this article.
It is a good idea to have a standard set of resets on these defaults before starting a project and very smart people have already written those for us.
A very famous one used by Twitter, Github, Medium, Bootstrap, CSS-Tricks, and others is normalize.css.
Just drop in the css code into your global CSS file before staring a new project and you should have all the css default reset and all the magically appearing margins and paddings should disappear letting us write predictable CSS code.