Fewer means a smaller number. Fewer cars on the road on a Sunday, fewer people at the second meeting, fewer mistakes than the last time you tried it.
This is the word people argue about. Fewer goes with things you can count and less goes with things you cannot: fewer coins, less money; fewer bottles, less wine; fewer cars, less traffic. Supermarket signs reading ten items or less are what start the arguments. No fewer than puts weight behind a number — no fewer than forty people turned up. Fewer and fewer describes a number that keeps dropping, and the superlative is fewest.
Fewer is few with the plain comparative ending -er stuck on the back, the same -er as in colder and older, and it was already written fewere in Middle English. Nothing more complicated than that ever happened to it.
Middle English
fewere
English, today
fewer
From Middle English fewere, equivalent to few + er.
Etymology from Wiktionary, CC BY-SA