Learn Hardcore Hausa: Idan kina so, za mu ci abinci tare da makwabta yau da yamma. - If you feminine want, we will eat with the neighbours this evening.

Idan is the basic conditional word meaning if (or sometimes when, depending on context).
In this sentence, Idan kina so = If you want.

You use Idan to introduce a condition, very much like English if:

Idan ka zo, zan tafi. – If you come, I’ll go.

There are other ways to express conditions in Hausa, but for a simple if-clause, Idan is the normal, very common choice.

In Idan kina so, what exactly does kina mean, and how is it used?

Kina is the 2nd person feminine singular progressive form, roughly you (female) are …-ing.
It usually comes before another verb or expression:

kina so – you (f) want / you (f) like
kina aiki – you (f) are working
kina magana – you (f) are talking

So Idan kina so literally has the idea If you (f) are wanting / if you (f) feel like it, which corresponds to If you want in English.

Why is kina used here? Does that mean the speaker is talking to a woman? How would it look for a man or for plural you?