A French plural is a plural that is formed in a manner typical of French. These are obviously, most commonly found in French itself, but many have been loaned into English via French loanwords and French loan phrases. In the UK at least, with French being the primary second language, educated people typically possess some knowledge of French.
Generally speaking, French plural nouns take the plural -s suffix, as English nouns do. However, in French this morpheme can become -x when inflected onto certain endings; -eau, -eu, and -ail changes to -aux.
In English, after loaning, these may be over-regularised to take an -s ending - this is non-standard (except for travails). In French, the final /z/ sound is not generally pronounced unless the next word begins with a vowel sound; in English the /z/ sound is optional.