Use the any of the entity selection techniques to select the entities to explode then select the explode button the selected entities are exploded. Hatching and Polygons are broken down into lines. Lines and Arcs are created from Polylines. Arcs are created from Ellipses. When an Area Fill is exploded the fill is deleted leaving the boundary broken into lines and/or arcs.
When an inserted Symbol is exploded the entities in the symbol are separated back into the entities used when it was created. i.e lines arcs, dimensions etc, however any entities which can themselves be exploded (other symbols, Area Fills, Polyline etc.) remain intact. Effectively the symbol is exploded by one level down, therefore when symbols are nested (e.g. Symbol1 contains a symbol called symbol2 which contains a symbol called symbol3) only the selected symbol is exploded any symbols within that symbol remain as symbols but can then be individually selected and exploded if required.