Sans-serif
Letters without finishing strokes at their ends, as in Helvetica or Arial.
See it in Text flow. Switch Font from Serif (Georgia) to System sans and compare the ends of the letters.
In plain words
Sans is French for without, so sans-serif means without serifs. The strokes end plainly, with no small feet or flicks. These letters look clean and modern and stay clear on screens. Most apps and phone interfaces use them. Helvetica and Arial are well-known sans-serif typefaces.