What is flavor development?
This is just fancy sounding talk for the myriad of techniques in good cooking that create good flavor in dishes.  It doesn't refer to any one specific thing.

Some examples of techniques used to develop flavor include:


Seasoning (properly salting)
Browning meats and other foods properly to bring out their flavor
Creating pan sauces or other uses from fond to enhance the flavor of a dish
Using herbs and spices skilfully to compliment the food
Balancing flavors with acidic ingredients like lemon juice or tomatoes
Cooking properly to allow the flavor of individual ingredients to shine through
Blooming or toasting spices to bring out their more aromatic qualities
Using alcohol to enhance certain flavors not soluble in water or fat


There are no doubt many, many more things that could be included in a list like this.

After all, it sounds far more professional to say "the flavor in this dish is not well developed" than to say "this dish is bad."