When making a decision, we often rely on reference points, or "anchors" and adjust from this point — a process that is often subconscious. We often rely too heavily on a single anchor though and our perception of the value of something can vary depending on what we have anchored to.
For example, estate agents often use anchoring to influence potential house buyers by showing them properties outside of their buying criteria (e.g. too pricey, but highly desirable) in order to influence how much they are willing to spend on other properties.
Anchoring doesn't just apply to money - for example experiments have shown that a person's perceived attractiveness really is influenced by the attractiveness of the other people in their group. Applied to market research, this means even small contextual cues or irrelevant experiences may anchor respondents behaviour.