Honorific term used to refer respectfully to someone else's husband. Not used for one's own husband.
ご夫君はお元気ですか。
How is your husband doing?
山田さんの夫君は医者だそうだ。
I hear that Ms. Yamada's husband is a doctor.
Compound of 夫 (fu, 'husband') and 君 (kun, honorific suffix). The reading ふくん is a standard on'yomi combination.