What are "Attributed Unique Households (Estimated)"?

Attributed Unique Visitors (Estimated) is the number of unique page visits to the advertiser's website that are de-duped down to the individual level. 

This figure is "estimated" because we perform direct measurement only on devices attached to a household IP address. This is the "confirmed" figure is in parenthesis. To calculate the the estimated number, we apply a multiplier, which extrapolates based on the assumption that the non-household devices performed at the same rate as the household devices.

More on this can be found in the article "What is the difference between "Household" and "Estimated" metrics?"