An arbor is a shelter of latticework made of a (typically wooden) trellised framework that is often covered with vines or some other climbing plant that has grown over the structure. The yard on Sesame Street featured an arbor outside the back door of Hooper's Store from season 2 to season 29 (after which time it was replaced by a community garden). Blueprints from the 1970s specifically identified it as a grape arbor.
Over the years, the term "arbor area" was sometimes used to refer to the larger space typically identified as the yard. Scripts and other internal Sesame Workshop documents used the terms interchangeably, which sometimes trickled into the narrative. For example, Big Bird tells Chris he'll wait in "the arbor" in Episode 4206 even though the arbor had been replaced by the garden at that point (notably in the same spot).
Although the action in Street scenes more frequently centered on the yard, the trellised arbor was notable as a location in its own right from time to time. Much of Mr. Hooper's night school studies took place in the arbor, including his study date with Margaret in Episode 0917 ("Oh, you have an arbor!").