One of the first things to learn about in Livecode is about stacks and cards. The names come from the metaphor of a stack of cards. There can be several stacks in your program, the first created is the Main stack, the others are the substacks. Each stack can hold many cards.
A card is a program window. If you display a card, that window and all its various controls are displayed. Card controls are all the various buttons, fields, graphics, groups, etc on a card. Each card can have a different set of controls. So if you display a different card, you change the window's appearance and what it can do.
This is another example: