Radicals are the parts of Chinese characters that give the character either meaning or sound. For example: 我 is made up of 手 (radical form:扌)and 戈 (a type of halberd): I am not sure of the exact etymology, but you can think of a hand holding a halberd is like an extension of ones arm (oneself) and perhaps takes the meaning of "I" for that reason. 在 is made up of 才 (cai; is used to provide the sound, which morphs to zai) and 土 (provides the meaning). Since 土 means "ground" you can think of, the ground is what you stand on and "where you are," so perhaps that is why it indicates location or existence. 很 is made up of 彳(provides meaning; it means go or step and comes from 行) and 艮 (provides sound "gen," but morphs to hen). Maybe you could think of taking a "step" forward as a kind of progression, just as the word 很 "progresses" the meaning of a word, since it means "very." Every character in Chinese is composed of various radicals and you will begin to tell what they are as you see the similarities and differences of words. You can also look them up in a dictionary to find their exact meanings and origins. I hope this helps. Please let me know if you need any more explanation on this topic.

In ancient times, people tried to communicate by drawing pictures. They drew something like fire to represent fire and eventually that picture became the word 火. Then, people wanted to express more complicated ideas and found that it was too difficult to draw pictures for everything, particularly abstract ideas. They came up with the idea of making a simple and common word part of a new word. So they created words like 燒, 烤 and 烘, which were cooking method using fire. Centuries later, people wanted to creat a dictionary to collect all the words to facilitate learning. But how could the words be classified for easy search? Then people came up with the idea of grouping the words by these common parts. So, 火 became one of these common parts which are now called "radicals".

It's something like suffixes or prefixes