Learn Turkish: Toplantının asıl konusu neydi? - What was the actual topic of the meeting?

In a genitive-possessive phrase like toplantının konusu (the meeting's topic), an adjective describing the possessed item (the topic) goes right before it. Therefore, toplantının asıl konusu means "the meeting's actual topic." If you put it at the very beginning (asıl toplantının konusu), it would mean "the actual meeting's topic."

Why does konu have the -su ending here?

The -su is the 3rd person singular possessive suffix. It pairs with the genitive suffix -nın on toplantının to form a definite noun compound. Together, they literally mean "the meeting's topic" or "the topic of the meeting."

How is neydi formed?

It is made up of the question word ne (what) and the past tense ending -di (was). Because ne ends in a vowel, the buffer letter -y- is inserted to connect them, making neydi (what was).

Could we use gerçek instead of asıl to say "actual"?

You could, but they have a slightly different nuance. Gerçek usually means "real" (as opposed to fake or imaginary). Asıl means "main," "primary," or "original." Here, asıl konusu perfectly captures the idea of the "main underlying topic" rather than just what was written on the agenda.