Attention to detail only has meaning when it gets through to the customer.
Literal
Obsession [topic-は] customer [to-に] conveyed [only-てこそ] meaning [obj-を] accomplish.
こだわり 'obsession / commitment to detail' can be positive (craftsmanship) or negative (stubbornness); here it's the positive sense of artisanal dedication. ~てこそ 'only when / precisely because' marks the indispensable condition — 'only when it reaches the customer.' 意味をなす 'to make sense / have meaning' is a set verbal phrase.