The term "bf" is most commonly an abbreviation for "boyfriend," meaning a male romantic or sexual partner. It is used often in internet slang, messaging, and casual conversations. Another common usage of "bf" is as an abbreviation for "best friend," referring to a person's closest friend. Additionally, "bf" has multiple other meanings depending on context, including references to places, people, science, and technology, but the two primary everyday meanings are "boyfriend" and "best friend".
If referencing photography, "BF" might also refer to specific camera models like the Sigma BF, or in gaming and other fields, it could have very different meanings.
For typical casual use, such as messages or online chats, "bf" means either "boyfriend" or "best friend" depending on context.