It's a hint or clue of what is to come. For example, say you're reading the book 'Clue' and in the first chapter it describes how Colonel Mustard was in the kitchen when the plumber came in to fix the pipes under the sink, and the plumber was messy and had tools and spare parts and plumbing pipes all over the floor. Then later, someone dies and they find a bloody pipe... you had that hint earlier that there were pipes in the kitchen.. and that Colonel Mustard had been in there. In some cases the foreshadowing is very obvious, in other cases you don't even realize the clue was given until AFTER the fact, which makes the conclusion even more exciting that you over looked it.
Foreshadowing is when an author drops hints throughout his/her writing that allude to a conclusion or a climax. In other words, if you're paying attention, the author subtly leaves you clues as to what's going to happen later on.