The basic idea is that the relative amount of space given to certain viewpoints and sub-topics within a given article must be represented proportionately to its overall importance. For example, the Wikipedia article "Apollo program" does not mention the moon landing hoax conspiracy theories until a late section on its cultural impact.
Generally, proper weight is determined by the availability of sources discussing a topic. However, this can be a challenge when news sources obsess over frivolous stories, like whomever Taylor Swift is dating at the time this is published.
It's an inherently controversial directive, as one historian found when his new scholarship was rejected from Wikipedia. For more efforts to think through the implications, see the guideline "Fringe theories" and essay "Reliable sources and undue weight".