-kah is used in making "Yes-No" types of questions. When we make this type of question in English, we invert the word order:

Statement: They are wrong. --- Question: Are they wrong?

But in Indonesian when you invert this sort of statement to form a "Yes-No" type of question, you add -kah to the predicate (in this case, that's salah) and invert the word order (usually):

Statement: Mereka salah. --- Question: Salahkah mereka?

An example using apa:

Statement: Ini book. (This is a book) --- Question: Apakah ini book. (Is this a book?)

And with ada:

Statement: Ada makanan. (There is food) --- Question: Adakah makanan? (Is there food?)

That is accepted now (as of mid-February 2019).