Dreams use two main mechanisms to disguise forbidden wishes: CONDENSATION and DISPLACEMENT. Condensation is when a whole set of images is packed into a single image or statement, when a complex meaning is condensed into a simpler one. Condensation corresponds to METAPHOR in language, where one thing is condensed into another ("love is a rose, and you'd better not pick it"--this metaphor condenses all the qualities of a rose, including smell and thorns, into a single image). Displacement is where the meaning of one image or symbol gets pushed onto something associated with it, which then displaces the original image. Displacement corresponds to the mechanism of METONYMY in language, where one thing is replaced by something corresponding to it. (An example of metonymy is when you evoke an image of a whole thing by naming a part of it--when you say "the crown" when you mean the king or royalty, for example, or you say "twenty sails" when you mean twenty ships. You displace the idea of the whole thing onto a part associated with that thing). You might think of condensation and metaphor as being like Saussure's syntagmatic relations, which happen in a chain (x is y is z), and displacement and metonymy being like Saussure's associative relations.
This work was, by his own assessment, Sigmund Freud's greatest. In the process of showing how seemingly meaningless fragments of dreams suggest the whole range of personal issues in the dreamer's present and past life, Freud lays out the basis for a new psychology and therapy. And anyone can use this book to know more about his life.