"Hide" is used to hide objects in the scene so that only the objects you are currently working on are displayed and edited. This option is very handy to keep object creation speed at its peak. It is also very useful in placing tightly grouped objects. "Show" will re-display the hidden objects. "Hide" option contains two sub-choices:
a. "Hide - Selected" will hide all the currently selected objects.
b. "Hide - Unselected" will hide all objects not selected. The latter is useful to hide away all objects that you are not working on.