is short for ‘STRONG RADICAL,’ which means ‘radical that usually controls the pronunciation of any kanji in which it is a component.’ For example, Both 可 (on-yomi: KA) and and 中 (on-yomi: CHUU) are STRONG.
So if you see a kanji with the 可 radical inside it, (i.e.苛、何, 河, or 歌 )most likely their ONyomi will also be KA. Likewise, if you see a kanji with 中 inside it, (i.e. 虫、忠, or 仲 )the ONyomi of that kanji will probably also be CHUU.
Strong radicals are good because they help you guess the ON-yomi of kanji that you can’t read.
But BE CAREFUL of kanji such as 己. 己’s ONyomi is KO, but most of the kanji which use the 己 radical have the ONyomi of "KI", not KO!!!!
In cases like that, I’ll write a warning tag which says something like: "STRONG RADICAL FOR ‘KI’"