Indeed, "sans-serif" is french and means "without serifs (ending lines)".
But more or less all fonts without serifs are modern, because serifs were used in "manufacturing" letters (especially already in times of stonemasons hammering letters into stone blocks) to give hints where a stem or bow starts.
Sans-serif fonts are often based on simpler geometrical elements, requiring a less forceful but rather delicate technique of production (drawing table instead of stonemasonry or metal carving).
Fonts not needing and not using serifs was a rather unfamiliar sight for readers, therefore they were also called "grotesque". With the computer-aided design of fonts, sans-serif font families became rather usual and are not grotesque anymore, just modern-looking.