'Look' as a noun can mean a particular style or fashion, or the appearance of someone or something specific.
It's not clear from your quotation whether it applies to the first part about phone cases, or to the second part about taking selfies, but "the look" is such a generic and flexible expression that it could apply to either! It could mean that your phone case will help you get your phone to look the way you want it to. It could also mean you want your photographs to look a certain way and that this phone app will give you the tools to achieve 'the look' you want.
Context is key, because on its own this expression is meaningless. It is often said in a 'knowing' way - ie you expect your audience to know what you mean.