Webopedia, here, describes it as a “small decorative line added as embellishment to the basic form of a character” and continues: “Typefaces are often described as being serif or sans serif (without serifs). The most common serif typeface is Times Roman. A common sans serif typeface is Helvetica.”
The picture below shows the letter A in Arial (a sans serif font very similar to Helvetica) and a capital T in Times New Roman (“the most common serif typeface”, as the previous paragraph notes). You can see the three “small decorative lines” on the points of the letter T. There are none on the capital A.
As last month’s piece mentions, serifs make text more readable, they “draw your eye” across the line, which is why body text is usually in a serif font. There is another reason for serifs, pre-dating the printing press. Stonemasons, when carving letters, would add little notches (or serifs) to decorate the edges of each character and to prevent the stone or marble from cracking at the end of each stroke. At least that’s what I read back in the 1990s, in some style guide or other, and although I can’t track that publication down, and have found nothing on the web to confirm this story, I’ll stick with it.
For the record, typefaces either have serifs or they don’t. You can’t add them to sans serif text or take them away from a serif typeface. I mention this because a delegate on one of my training courses many years ago spent rather a long time trying to find the option in the software program we were using (probably Ventura, when it was still owned by Xerox). She had tried every item on every menu and gone through every dialog box before asking me where it was. It wasn’t anywhere. There was no option for it. This only happened in one training course but I made sure to mention it in every subsequent course, just in case. You shouldn’t spend too much time searching for something that isn’t there.