What is meant by an act plays

Act: Dominant structural unit in the drama, which guarantees spatial and content structuring. The act can be further subdivided into scenes or appearances.
The older term "elevator" corresponds to the "act" that is customary today, but still contains the original connotation of a change of scenery with the associated opening and closing of the stage curtain between the acts. It was not until the late 18th century that curtains made it possible to change the scenery in the nude that the dramatic and dramaturgical function of the nude emerged as a stage in the course of action. Characterized by a certain coherence in terms of content, however, it is further divided into scenes or appearances.
Subsequent to the ancient demands of Horace and Varro and the mediation via Italy and France to Germany, five acts is the norm in the classic form of drama. Each act was assigned a specific function within this closed whole. The scheme developed by Gustav Freytag of exposure - increase - turning point / periphery - retarding moment - catastrophe / solution can be regarded as exemplary. (see closed form). Although this was not created until the 19th century, it was precisely at this time that other models were widely accepted, for example the three-act act prevailed above all in the boulevard theater or opera, whereas four-act acts can often be found in the theater of naturalism. At the turn of the century, one-act plays were often created, and in the 20th century the nudes were often replaced by stations, episodes or pictures connected in loose succession.
Scene: Structural unit of the drama that divides the act and describes the events between two changes of scene.
Originally (and often still today in theater practice) 'the scene' referred to the stage design of the drama (Greek: Scena = scene), from which the name for the happening between two changes of scenery or location results. However, the term is often used synonymously with "appearance", although in the strict sense a scene can comprise several appearances. In the course of Shakespeare's reception in Germany in the 18th century, dramas with numerous changes of scene were created, for which the subdivision into scenes was much more appropriate than that into appearances. The stage works of Sturm und Drang in particular, which may also allow longer time intervals to pass between the individual scenes, make use of this type of division. As an example, the dramas by J.M.R. Lenz are the early dramas of Goethe or Schiller robber.
Appearance: Structural unit of the drama that divides the act and describes the happening between two changes of person.
Originally, appearance referred to the first stage presence of an actor, from which the name for an action unit with constant staff results. Especially dramas with few changes of location and those that adhere to strict rules for leading people work with the division of the act into appearances.
The rule stems from Horace that no more than three people should speak during a performance. In addition, there is the requirement from French classicism that the stage should never be empty and that the departure of one character should not be the appearance of another. As a result, short appearances have to be inserted over and over again in order to transition from the conversation between two people to that of two others, even if the conversations take place in the same room.