A radical (部首) is a component used to sort characters in a dictionary. 木 appears in 林 and 森 and is a common semantic hint for wood or trees. 氵 in 河 and 海 points to water.
Radicals are a teaching and lookup device. They are not a full theory of every character. Some "radicals" in a modern index are just the leftover part after you cut the graph the way the dictionary wants.
Conversion
OpenCC does not convert radicals by themselves. It converts complete characters. Knowing radicals still helps you notice when a conversion looks implausible: a water radical that vanished, or a form that no longer matches the word family.
Continue with how characters are constructed.