Symmetrical balance is mirror image balance. If you draw a line down the center of the page, all the objects on one side of the screen are mirrored on the other side (they may not be identical objects, but they are similar in terms of numbers of objects, colors and other elements. Sometimes they are completely identical (often seen in architecture).
A good example of this can be seen in this hosta leaf found in my side garden. You can draw a line down the middle of the leaf, and everything on either side would be mirror image.