Critical appreciation is when a reader examines and evaluates a piece of literature from a discerning point of view. In other words, the reader knows the work well enough to have an intelligent basis for liking or disliking something. Let me give you an example.
If you say that you like the works of Sophocles, the Greek author of tragedies, then a critical appreciation of Sophocles will give reasons why you like him. You can say that you like his poetry style, his use of language, and the like. You can also say that you like his character development and his themes. You can say that he work is applicable to life. You can also write what you find difficult and not so favorable as well.
The key point is that you know the works well enough to give an in depth reason for your judgments.