A radical (bushu) is a common sub-element found in different kanji characters. Every kanji has a radical or a radical itself can be a kanji. Radicals express the general nature of the kanji characters. A radical is the part of the kanji character that gives you a clue to its origin, group, meaning or pronunciation. Many kanji dictionaries organize characters by their radicals.
There are 214 radicals. Don’t worry about learning all of them! I doubt most Japanese can recognize and name them all. However, if you memorize some of the important radicals, they will help you to figure out the meanings of many kanji.
Radicals are roughly divided into seven groups (hen, tsukuri, kanmuri, ashi, tare, nyou, and kamae) by their positions.