Learn Spanish (Spain): ¿Entiendes al jefe? - Do you understand the boss?

This sentence uses the "personal a", which is required in Spanish when the direct object of a verb is a specific person. Since the boss is a person, we need a before el jefe. When a and el are next to each other, they must combine to form al.

Would I still use "al" if the boss was female?

No. If the boss were female, the word would be la jefa. The personal a only combines with el, not la. So you would keep them separate and ask: ¿Entiendes a la jefa?

Why is the verb "entiendes" instead of "entendes"?

The verb entender (to understand) is a "stem-changing" verb. For the tú (you) form, the middle e changes to ie, making it entiendes.

How do I know this is "do you understand" and not just "you understand"?

In Spanish, you don't need extra helper words like "do" to ask a question. The upside-down and regular question marks around ¿Entiendes? are what turn the statement "you understand" into the question "do you understand?".