To explicitly name something or state a particular detail, you specify that thing. So, when you have a craving for dessert and you send someone to the store to buy ice cream, you may want to specify a flavor.
If you assign something for a particular purpose, this verb means that you specify. Your parents can specify for safety's sake that you can only ride your bicycle on the sidewalk and not on the street. Another use of the verb specify is to give a set of requirements. If you have your grandmother's old recipe for snickerdoodle cookies, you may wonder what the recipe means when it specifies that you should add "a smidgen of molasses."