What is this?
To enable visual searching of Kanji, the grid above allows you to select various Kanji radicals (each kanji is made up of one or more radicals - the kanji itself may even be a radical). By selecting these parts, the search will find all the Kanji that contain the selected radicals. Radicals are grouped by stroke count, pressing these buttons will scroll you to first radical with that amount of strokes.

So what is a radical?
A radical is a component or part of the kanji; for example, the kanji 字 is made up of the radicals 子 and 宀.