A "while" statement has the form
while (<expression>) <statement>
As long as the expression is true, the statement (which can also be a code block) is executed. Be careful with your while loops! You can easily make infinite loops, in which case your game will hang and not react to any user input anymore. Below you can find an example of a typical way to use "while":
while (!place_free(x, y))
x = random(room_width);
y = random(room_height);
The above program tries to place the current object at a free position (this is about the same as the action to move an object to a random position).