This is another assignment statement. The RHS is the counter, which is an integer. The LHS, however, is a bit more interesting. The LHS, Command0.caption, specifies “the caption property of the button Command0”. The period (.) means almost the same thing as apostrophe-S in English.
It is rather interesting that Command0 is not only a variable by itself, but it also contains smaller components. This statement only affects the caption property of Command0. As soon as this statement executes, the caption of the button is changed.