Metaphor is only one kind of trope. Trope is any rhetorical technique that describes something in non-literal ways.
For example, metonymy is a technique where one word is replaced with a phrase that is related but is literally different, such as “the law” for police.
Synecdoche is using a part to describe the whole: “Can you give me a hand?”.
Irony is the use of words with opposite meaning, such as saying “You’re looking well.” to someone who is clearly quite sick.
There are also some expressions that are not considered as fully fledged tropes but as sub-tropes.
Here are two websites to get started on tropes: http://users.aber.ac.uk/dgc/Documents/S4B/sem07.html http://www.academia.edu/3793667/The_Fourth_Master_Trope
(The latter website may explain the information you have been given: “The late-twentieth-centurywidespread reduction to one master trope, metaphor, especially under the influence of Lakoff and Johnson (1980), is the most radical (and absurd) of these projects.”