The document discusses stacks, which are linear data structures that follow the LIFO (last in, first out) principle. Values are inserted into and retrieved from one end, called the top of the stack. The two main operations are push, which inserts a value into the stack, and pop, which retrieves a value. An example C program demonstrates these operations on a stack implemented with an array. The stack starts empty and grows as values are pushed on until it reaches its maximum size, at which point it is full. Values can be continuously popped off until the stack is empty again.