Same as with "fewer" and "less". Although, it doesn't seem as important in English anymore. "Fler/färre" is used with countable nouns, "Mer/mindre" with non-countable nouns. Färre = fewer. Mindre = Less. "Mindre" can also mean "smaller" so when used with countable nouns it means "smaller". "people/personer" is countable. So, "more people = fler personer" and "fewer people = färre personer". "Mindre personer" would mean "smaller people". A lot of Swedes mix up "mer/fler" though. "Mer" is often used when "fler" should be used.