The Brief: Hater means "naysayer."

DEEP DIVE
A hater is a critic, a skeptic, or a pessimist, with negative feelings usually targeted at someone or something specific. They are similar to trolls. The word is also seen stylized as “h8r.”
Haters are often talked about in conjunction with the idea of rising above negativity. “Rise above the haters” and “haters gonna hate” are commonly used phrases meant to encourage positivity.
Haters are also said to drink “haterade,” a punny combination of the words hate and Gatorade. The image of drinking a beverage underscores the idea of haters filling themselves with antagonism.
Haters will broadcast your failure, but whisper your success
— Knowledge (@TheKnowledge) January 30, 2019
TB12 has the perfect message for the haters.#SBOpeningNight | #EverythingWeGot pic.twitter.com/5RqXkHMl66
— New England Patriots (@Patriots) January 29, 2019