Merriam-Webster is a popular a publisher of American-English dictionaries: emphasis on American. It also had a significant role in shaping American-English as distinct from British-English. E.g. removal of the letter u in British-English words like colour. Or, the unsuccessful campaign to spell the word acre as aker.
That's false. Webster is the dictionary that had a role in shaping American English in a distinct fashion. The quality of the work deteriorated when the Merriam brothers purchased it, and they changed the title from Webster's by the time they had begun compromising the value of English reform.