In French (and sometimes so used in English), the term refers to the inappropriate use of anglicisms (English words) for which there are more appropriate French equivalents.
It can also mean a mangled combination of English and French, produced either by poor knowledge of one or the other language or for humorous effect. If you try to speak French and fill in gaps in your knowledge with English words or false friends with their incorrect meaning, the result is Franglais.
The existence of a franglais dialect is the result of the long-lasting coexistence of two linguistic communities inside Quebec and especially the Montreal area. Since the rise of Quebec as a society mainly controlled by its French-speaking majority in the 1960s, franglais is pouring into the dialects of Quebec anglophones who are increasingly bilingual. It is today frequent to hear anglophone Quebecers perform code switching (towards French) in the middle of a sentence like French Canadians used to with English not so long ago.
Alternatively, Franglais can work in reverse to provide less-than-ideal translations.