Summoning is a method in which a lucid dreamer can bring an object, be it a person, money, a bite to eat or whatever, into the dream, of their own intentions. It is one of the foremost lucid activities.
Although summoning is going against the natural flow of the dream, it need not be evasive; a dreamer can construct the objects appearance so that no evidence of extraordinary circumstances governing its creation are apparent, for instance, a dreamer can will an object to appear in another room, walk there, and thus obtain the object inconspicuously.
The most basic form of summoning is just to want an object to appear either in the dreamer's hands or in front of them. This is a perfectly valid technique, but one that requires a high level of lucidity. There are other, more elaborate techniques available; follow the links in the See Also section for more of them.
Summoning an object into a dream can be incredibly useful. For example, if one comes across a locked door, one can summon a key to appear in his pocket, and unlock it. Alternatively, you may wish a specific character to accompany you in a dream; they can be summoned also. And one of the more common objects summoned is food, to satisfy hunger, and money, to satisfy a less fundamental desire.