Refers to the French language. Often used in contexts like 仏語を話す (speak French) or 仏語の新聞 (French-language newspaper).
He is fluent in French.
I'm taking a French class.
This letter is written in French.
He speaks French fluently.
I bought a French-language newspaper in front of the station.
仏語 refers to French while 独語 refers to German; both are kanji abbreviations built from the first character of the country's name, so learners can mix up which kanji stands for which country.
Abbreviation of フランス語 (Furansu-go), using the kanji 仏 (Futsu) as an ateji for 'France'.