Layout is a term used to refer to the general layout of the game. Most of the time, it refers to the markings printed on felt or the cloth that covers casino game tables in brick-and-mortar casinos.
If you have ever sat at a baccarat table, for example, you must have noticed the different markings on the table and different areas that indicate where the cards go, how and where to bet, and so on. Usually, the layout varies from one game to another, but sometimes it can also vary from one casino to another.
If you are playing online, the term layout can sometimes be used to refer to the user interface. All in all, whenever someone is talking about the layout of the game, they usually refer to everything you can see on the table, such as markings, betting areas, etc.