This is the second sentence in your Introduction Paragraph. The purpose of this sentence is to connect or bridge the Hook Sentence with the Background Information. This will increase the flow of your ideas and make the essay easier to follow for the reader.

The Hook introduces the idea / topic / theme of your essay in a general and interesting way. The Bridge basically states that this idea / topic / theme exists in the work (novel, poem, play, etc.) that you are referencing. The Background Information then provides the reader with the main details about the referenced work they need to understand the idea / topic / theme of your essay. 

Example

If you are writing an essay about what makes Santiago a good example of a role model in The Old Man and the Sea:

A role model is a person looked up to by others as an example to be imitated and inspired by. (Hook Sentence – Definition) Manolin, the young man in The Old Man and the Sea, is fortunate to be positively influenced by Santiago, the main character in the novella. (Bridge Sentence stating that the idea of a role model exists in the novella) Ernest Hemingway’s (who) 1951 (when) story reinvigorated his career (why) and follows a veteran fisherman named Santiago (who) from Cuba (where) who has been unable to catch a fish for 84 days. Santiago is finally able to catch and kill a marlin but eventually loses the fish to ravenous sharks (what). (Background Information of the novella – 2 sentences)