From Design Pattern by GoF:
Another useful thing to show is which classes instantiate which others. We use a dashed arrowheaded line to indicate this, since OMT doesn't support it. We call this the "creates" relationship. The arrow points to the class that's instantiated. In Figure B.lc, CreationTool creates LineShape objects.
What does "a class instantiates another" mean here?
Thanks.

Another way to phrase it would be "a class creates an instance of another", which makes it a little more obvious.
It just means that
Class Acreates an instance ofClass Binside of it (or "Class AinstantiatesClass B").For example, you might have a
Personclass, which creates an instance of aHandclass, which it then uses.