How clues work
A clue is a plain fact about where one person was: "She was in the Library." "He was beside a plant." No clue is a lie and none is decoration. Each one leaves that person a set of cells where the fact holds, and you cross out the rest.
Clues come in positive and negative forms. A positive clue such as "in the Library" usually leaves a handful of cells. A negative clue such as "not in the Kitchen" removes a few and leaves the rest open, so it rarely solves anything alone and is often the clue that settles the last choice.
Two clues, alone in the room and with another person, talk about who else shared the room. They work on counts, so they get stronger as the board fills. Combine every clue with the one rule from sudoku, one person per row and per column, and the board has exactly one solution. The exact wording of each rule is on the rules page.