It means "great," "large," "powerful," or "important." This is why we see it in words like:
* Magnificent: A combination of "Magnus" and "fic" (to make), meaning "to make great."
* Magnify: To make larger or more important.
* Magnitude: The size or importance of something.
* Magnate: A powerful or influential person.
The word "Magnus" itself is still used in some contexts, particularly in scientific and technical fields, but it's more common to see its derivatives in everyday language.