Subcommentaries are like super detailed explanations about the teachings of Buddha. It's almost like a commentary on a commentary! These subcommentaries were written by Buddhist monks and scholars a long time ago, so that people could better understand the teachings of Buddha. The subcommentaries talk about things like the meaning of certain words, the history of certain teachings, and even some debates that happened between monks about certain ideas!
Theravada is a type of Buddhism that started in southern Asia a very long time ago. Theravada literally means "the way of the elders." This type of Buddhism really focuses on the teachings of Buddha himself, and places a lot of importance on studying the Buddha's words and trying to live your life in a way that follows his teachings.
So, subcommentaries are like really detailed explanations of the teachings of Buddha that help people better understand what he meant. And Theravada is a type of Buddhism that really focuses on studying and following the teachings of Buddha himself.