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.
Cards, Stacks and Substacks diagram
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:
Stacks and cards structure