The importance of Robinson Crusoe to English Literature is that it is considered to be one of the most important precursors of the novel as a genre (the others being Defoe's "Moll Flanders," and "Roxana" and a number of other fictions written by writers now forgotten, among them Mrs. Eliza Heywood.
Robinson Crusoe is NOT a novel because there is no development of character, and no central conflict to be resolved. Crusoe is marooned on an island for twenty-four years, left to its natural resources and his own industry for many years (until he finds Friday), establishes a two-person fiefdom by conquering the other inhabitants of the island and eventually returns home. (see the enotes summary)
It is an important precursor of the novel mainly for one reason: it is a thorough attempt to write a realistic fiction.
If you read the section on the setting of this story in the enotes "Study Guide to Robinson Crusoe," you will realize the maximum importance Daniel Defoe gave to things like trade and economics while writing this book. In other words, he was interested in "real things." This emphasis on reality is what distinguishes Defoe's fictions and the works that followed (like Richardson's Pamela, Clarissa, Fielding's Joseph Andrews and Tom Jones) from all the other preceding works that told a story.
To Defoe, the purpose of writing these books was plain and simple: to instruct readers about economic reality and to teach them sound moral principles. Thus, the work is filled with hundreds of little details, from how Crusoe brought goods back from his wrecked ship, to how he built himself a make-shift house. Indeed, these details begin to get to you and there is a tendency is to skip over them. But the cumulative effect of these details is precisely what Defoe wanted: an appreciation of reality, and, hence realism,one of the fundamental features of all the novel. They are different from epics and romances in the sense they are rooted in reality.
The other realistic feature of Robinson Crusoe is religion. The enotes Study Guide gives an excellent summation of the religious context of Robinson Crusoe so I won't go into it in great detail. Suffice it to say, that in the first quarter of the eighteenth century, Christianity, especially among the dissenters, was of utmost importance. A work of fiction should certainly entertain (the Latins called it "dulce.") But it was also expected to instruct morally, "utile." In fact, to men like Defoe, religious and moral instruction was as critically a part of realism as economic reality.
Finally, plot. According to the critics of eighteenth century literature development of plot was not a consistent factor in fiction. Plot was much more a concern for Defoe in Moll Flanders or Roxana, than it was in Robinson Crusoe. Later, when the novel as a genre was established by people like Richardson, Fielding, Jane Austen (to name only a few), plot and structure became a regular thing.
Thus, if you were to write an introduction to Robinson Crusoe, be sure to mention: 1) that while it is not a novel, it is an important precursor to it, perhaps the most important one; 2) it contributed greatly to the single most important feature of the novel, viz., realism; 3) since the novel as a genre is an important byproduct of England's economic environment in the eighteenth century. Robinson Crusoe, by emphasizing the economics became a landmark of English Literature.