It is the part of a Chinese character indicating its general semantic classification, to which group it belongs, sometimes called a classifier, in Chinese 部 首 （bu4 shou3) . The number 3 indicates that the word is pronounced at the third tone.

Example : the radical heart 心 / xin1

The following five words are written with the radical heart

To think 想 xiang3

To feel 感 gan3 / g in pinyin has the phonetic value of k

To deliberate (inside your mind) 思 Si1

Angry 怒 nu4

Idea/Thought/Meaning 意 yi4

You can see the component 心 in all 5 logographs and they pertain to the fields of feeling or thinking, the heart 心 Xin1.

There are 214 official radicals since the Kangxi Emperor's dictionary produced in 1716.

Before it, older dictionaries gave between 300 to 600 radicals.