CSS Reset
What it is
Every web browser has a built in set of CSS rules that are applied by default to every HTML document, called the user agent stylesheet. That is why when you look at a “raw” HTML document, it will still have some basic styling like large headers and blue, underlined links:
Unfortunately, browsers have slight differences in their user agent stylesheet, and due to the CSS Cascade, that can lead to inconsistencies in how your page renders in different browsers.
To mitigate this, you should use a CSS Reset, a small minified set of CSS rules that reset these user agent rules to a consistent baseline.
There are many resets available, and as far as I care to discern, they are all basically the same. I stick with Normalize.css in my projects.
Why it matters
Without using a CSS reset, your page may look different in different browsers, which is a headache.
Related guides
How to learn it
You just did. take a look at the source of one of the common resets to see exactly what they’re doing.