IIFYM is If It Fits Your MACROS.

It refers to the idea of eating whatever food you like, so long as it fits your overall caloric intake and macro breakdowns for your goal.

IIYFM assumes all calories are created equal. This is not true though, because, for example, you do not get the same nutrients and such from CARBS in a pop tart as you do from sweet potatoes.

You should be focusing on QUALITY of food, not just QUANTITY. Junk food may cause an insulin spike from sugar, and insulin is the main fat storage hormone as well as the hormone that forces nutrients into your cells.

Related Questions:

1. What is the best way to split up my macro percentages?
2. Are carbs really bad?