Tooltips are the little comments that appear when the cursor rests on some of the buttons of CalGraph. Don't click, just wait a moment. If the tooltips do not appear, they may have been turned off. Click Customize at the top of the window and then Restore Tooltips.
If you don't like them, click Customize at the top of the window and then Remove Tooltips.
For each function button, the tooltip indicates the Domain and Range. For example, hold the cursor over the button, and the yellow tooltip says Domain = (0,infinity), Range = all. In English this says that Log is defined for numbers running from 0 to infinity (that is the positive numbers) and its output can be any number at all. This can be helpful if you try to plot a graph and nothing appears, just the message Out of Bounds. It can also suggest the problem if only part of the graph you expect is visible.