Why does lietentant kotler mean by spare tire and what does Bruno think it mean?

Lieutenant Kotler uses the term "spare tire" as a metaphor for someone who is unnecessary or extra, suggesting that certain people can be easily replaced or do not add value to a situation. Bruno, however, misinterprets it, thinking it refers to a physical object, reflecting his innocence and naivety. This highlights the different perspectives and understandings of the characters in their respective contexts.