The Merriam-Webster dictionary defines foreshadowing as follows: "to give a suggestion of something that has not yet happened." Foreshadowing is a common literary device, and is used often in many books, including The Great Gatsby.
In The Great Gatsby, Fitzgerald uses foreshadowing on many occasions. One such example occurs in Chapter 4 when Nick is introduced to Mr. Wolfsheim, a gambler who is friends with Gatsby and who, according to Gatsby, "fixed the World's Series back in 1919" (Fitzgerald 73). This meeting is an example of foreshadowing to the reader that Gatsby obtained his wealth through illegal means rather than legitimate business, something the reader learns to be true later in the book.
Another example, that occurs later in this same chapter is when the reader learns that Gatsby is in love with Daisy and wants to plan a meeting at Nick's house as an excuse to see her. Jordan Baker tells Nick that Gatsby wants Daisy to come to tea at Nick's house because "He wants her to see his house" (Fitzgerald 79), which is right next door. This is another example of foreshadowing, suggesting to the reader that Gatsby believes he will be able to win Daisy's love because he is now wealthy like her husband Tom Buchanan. This foreshadows the idea that Gatsby believes Daisy only married Tom for his money, and not because she loved him. This reinforces to the reader Gatsby's belief that Daisy will leave Tom for him, an important theme later in the book.
As one can see from these examples, Fitzgerald conveys to the reader important information that is later confirmed in the book through the use of foreshadowing. Hope this helps!