A context menu is a pop-up menu whose content depends on the context in which it is invoked.
For example, right clicking a single selected word in a word processor might cause a pop-up context menu to appear that includes operations that are appropriate to apply to a word. Right clicking on a complete paragraph that has been selected might show paragraph-specific operations. Right clicking in a document might show a completely different set of options in the pop-up context menu.
In each case the context surrounding where and when the click happens is used by the software to determine what the menu should contain.
Context menus are often called “pop-up” menus, because they appear at the location the mouse is clicked or right clicked, rather than being attached to a menu bar.