This is the 7732nd most frequent Russian word.
зубр
European bison.
In this sentence, 'зубр' is used to designate the European bison, an animal species.
Это зубр, крупнейший наземный млекопитающий Европы.
This is a bison, the largest terrestrial mammal in Europe.
Here, 'зубр' is used metaphorically to describe someone with exceptional expertise in a field.
Мой сосед — настоящий зубр в математике.
My neighbor is a real expert in mathematics.
In this usage, the verb 'зубрить', derived from 'зубр', means to memorize through repetition, implying intense studying.