The Grid is the area of the page primarily given over to visual content – images, video, etc. Your options here will vary widely from one plugin to the next, but some common options always apply. The entire Appearance control pane is given over the customization of the grid.
You may always choose to enable/display the grid. If enabled, you may use the provided controls to set the background-color, width, borders, spacing, etc. for the grid area on the page.
Beyond these basic layout options, you will find additional options depending upon the intent of the plugin. For image galleries, you will be able to configure the appearance and behavior of the gallery. For pages, you will find controls allowing you to insert an image, or leave the area blank for PHPlugins implementation. In TTG CE3 Stage, you will find options for embedding video and other types of content. See individual plugin documentation for details of the available features.