Complex sentence: Mets rivalry is a rivalry between two teams in Major League Baseball's National League East, featuring the Atlanta Braves and New York Mets, that was particularly fierce during the late 1990s and early 2000s.
Split into simpler sentences: Mets rivalry is a rivalry between the Atlanta Braves and New York Mets. The rivalry between the two clubs was particularly fierce during the late 1990s and early 2000s.