To separate groups, instances, and bitmaps into ungrouped, editable elements, you break them apart, which significantly reduces the file size of imported graphics.
Although you can select Edit > Undo immediately after breaking apart a group or object, breaking apart is not entirely reversible. It affects objects as follows:
Severs a symbol instance’s link to its master symbol
Discards all but the current frame in an animated symbol
Converts a bitmap to a fill
Places each character into a separate text block when applied to text blocks
Converts characters to outlines when applied to a single text character.
Do not confuse the Break Apart command with the Ungroup command. The Ungroup command separates grouped objects, returning grouped elements to the state they were in before grouping. It does not break apart bitmaps, instances, or type, or convert type to outlines.