Not a soap cap. Soap caps usually contain bleach and a developer. They are used to remove unwanted colors or tones. You are referring to a gloss or glaze. If you are not wanting to have your hair "processed" in any way be sure of what you are purchasing. Here is the definition of both.
A gloss is a bit more permanent than a glaze. It adds shine and adjusts the tone (say, if your highlights are too bright or brassy, a gloss will subdue them). A gloss penetrates the hair's cuticle, so it lasts two to four weeks. A glaze, on the other hand, simply coats the shaft with shine and semipermanent color; it's like putting a top coat of polish on your hair, and it lasts a week or two.
Hope this helps!