What it means
か is a sentence-final question marker. It turns a statement into a question and is commonly used after polite forms such as です and ます. It can ask for information or confirmation, and the expected answer is often yes or no. Sentence-final か after a plain form is grammatical, but it is not generally a neutral familiar-speech pattern; it can sound formal, literary, blunt, or masculine depending on the context.
Usage note
In ordinary familiar conversation, speakers usually make a question with rising intonation and omit sentence-final か. Therefore, plain-form questions such as 行くか are marked and should not be treated as the usual familiar counterpart of polite questions such as 行きますか.
Example sentences
この電車は新宿へ行きますか。
Does this train go to Shinjuku?
明日は休みですか。
Is tomorrow a day off?
あの建物は図書館ですか。
Is that building a library?
この料理は辛いですか。
Is this dish spicy?
駅の出口はあちらですか。
Is the station exit over there?