A trademark application must specify the goods or services it covers because trademark protection is class-specific. The registration grants exclusive rights only in connection with the goods or services listed. A competitor using the same mark in a different industry may not infringe; a competitor using it in your registered class will.
The goods and services list also defines the basis for distinctiveness assessment. Whether a mark is registrable depends in part on whether it is descriptive of the specific goods listed. A word that describes the goods directly is refused; the same word used in connection with unrelated goods may be registered as an arbitrary mark.