Upādhi (superimposition or limiting adjunct) is a thing which communicates its own property to another that is situated close to it. For instance, when a red flower is placed near a crystal (glass), it imparts its red color to the glass which then appears red. In this case, the red flower is the Upādhi for the crystal. In the same way, ignorance (present in all individuals) imparts its property of unconsciousness to Knowledge (or Brahman which is present and close to the Jīva in each person) so as to render it separate and dual.
It is important to note that, whereas the crystal, was not visible by itself (not likely visible in isolation unless a colorful object is placed in association with the crystal to render it visible) the Upādhi helps us see the crystal.
Similarly, a safety sign placed on clear glass doors helps people avoid bumping into the glass door. The safety sign which helps to see the glass as a glass door is the Upādhi for the glass. Mud is in all pots, yet, if the pot is broken, mud is not impacted; mud is still mud (the name and form is lost, yet the substratum is not affected). So, the pot is not in mud. The pot is an Upādhi of mud.
In the same way, the names and forms of jagat (manifested world) are Upādhi for Brahman. Brahman is able to be identified only through the Upādhi of names and forms. If Brahman had not created the jagat, we would not have known its svarūp (true nature). Therefore, jagat serves as an upādhi for Brahman to be known. Yet, Brahman is not affected by the awkwardness of jagat, just as gold is not affected by the ring or, a crystal is not affected by an association with a colored object near it.