A non-essential expression is the one that can be omitted without altering the meaning of the sentence.
For example, in "I spoke to Mr. Smith, my favorite teacher, about my essay" we could remove "my favorite teacher" and the meaning remains the same. Usually, the non-essential expression is placed between commas.
Due to the coronavirus public health emergency, the John F. Kennedy Presidential Library and Museum will be closed through December 31, 2020 and until further notice. More information.
