In English language punctuation, a serial comma or series comma (also called Oxford comma and Harvard comma) is a comma placed immediately before the coordinating conjunction (usually and or or) in a series of three or more terms.Much more at the link.
For example, a list of three countries might be punctuated either as "France, Italy, and Spain" (with the serial comma), or as "France, Italy and Spain" (without the serial comma)... It is used less often in British English, but some British style guides require it, including The Oxford Style Manual...
The style that always uses the serial comma may be less likely to result in ambiguity. Consider the apocryphal book dedication quoted by Teresa Nielsen Hayden:
To my parents, Ayn Rand and God.There is ambiguity about the writer's parentage, because Ayn Rand and God can be read as in apposition to my parents, leading the reader to believe that the writer claims Ayn Rand and God are the parents. A comma before and removes the ambiguity: To my parents, Ayn Rand, and God.
An example collected by Nielsen Hayden was found in a newspaper account of a documentary about Merle Haggard:
Among those interviewed were his two ex-wives, Kris Kristofferson and Robert Duvall.The Times once published an unintentionally humorous description of a Peter Ustinov documentary, noting that "highlights of his global tour include encounters with Nelson Mandela, an 800-year-old demigod and a dildo collector". This would still be ambiguous if a serial comma were added, as Mandela could then be mistaken for a demigod, although he would be precluded from being a dildo collector.
In her style guide Eats, Shoots & Leaves, Lynne Truss writes: "There are people who embrace the Oxford comma, and people who don't, and I'll just say this: never get between these people when drink has been taken."
Related: Is the semicolon an endangered symbol?