Is visiting a parent’s grave Halacha? If so, can you give me the citation of the law? If it is only custom: What is the custom? When and how did it originate? What is the religious significance?

Kav Hayashar 71 states that it is a good custom to visit parents' graves on the Yartzite day. In Yerushlaim the custom is to go also on the day before Rosh Chodesh Nisan and Elul. However Rabbi Moshe Feinstein in Igrot Moshe YD2 162 wrote there's no obligation to do so and it seems he hints that people hold highly of this custom more than it really is.