Final answer:
When a background program attempts to read from STDIN, its execution is typically suspended because it doesn't have access to the terminal's keyboard input.
Step-by-step explanation:
If a program executing in the background attempts to read from STDIN, what usually happens is its execution is suspended. This is because by default, the standard input is connected to the terminal's keyboard input. With the program in the background, it does not have access to the terminal, and when it attempts to read from STDIN, the shell suspends it. This state is commonly referred to as being 'stopped' and the process will remain in this state until it is either brought to the foreground, where it can read input from the terminal, or is provided input through other means, such as redirection or a pipe.