Cannot see description when hovering over icons

I used to be able to see the description of icons when I hovered over them with my selection tool. For instance, I could hover over the different Pathfinder icons and little pop up descriptions would say "merge", "divide", etc.  Or for instance, when I hover over the Align icons, it would say "horizontal align", "vertical align", etc.  However, I must have pressed something because I can no longer see the pop up descriptions when I hover my selection tool over icons.  Please help.  .