This is the 1104th most frequent Turkish word.
değildi
"Değildi" means "was not" in English.
In this sentence, 'değildi' is used as a negation for an adjective, implying that 'güzel' (nice) does not describe the subject (the weather) in the past tense.
Bugün hava güzel değildi.
Today the weather was not nice.
Here, 'değildi' negates the temporal phrase, conveying that the specified time does not apply to the meeting in the past tense.
Dün buluşmamız saat dörtte değildi.
Yesterday, our meeting was not at four o'clock.
In this case, 'değildi' negates the comparison idea, indicating that the book did not meet the degree of interestingness anticipated, in the past tense.