A CSS reset is short compressed/minified set of CSS rules that resets the styling of all HTML elements on a page to a consistent baseline.
Every browser has their own user agent stylesheet to make unstyled websites more readable.
Ex: Most browsers make unvisited links blue and underlined and visited links purple, and apply different sizes to H1, H2, etc...
With CSS reset, developers can force browsers to reset all default styles to null to avoid cross-browsers differences.
Example of tool: Normalize.css
Some developers argue that css resets or normalizers are a bit useless and the same thing can be done with some CSS rules without having to add an extra stylesheet.
This article shows a few screenshots of website with and without CSS resets. The difference is sometimes so small that the use of a normalizer is not needed.
