だった
was, were (casual)
Noun / な-adjective + だった
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だった is the past of だ, and the casual version of でした: 昨日は雨だった, "it rained yesterday".
Like だ, it goes after nouns and な-adjectives: 静かだった, "it was quiet". い-adjectives never take it: the past of 高い is 高かった, not 高いだった.
You'll meet it constantly in novels and manga narration, which are written in plain style even when the characters speak politely.
It also turns up inside longer sentences that end politely, because the part before から, けど or a noun is usually plain: 雨だったから、行きませんでした, "it was raining, so I didn't go".
Register: Casual. The polite version is でした.
Examples
昨日は雨だった。
It rained yesterday.
子どもの時、野球が好きだった。
I loved baseball as a kid.
昨日の町はすごく静かだったよ。
The town was really quiet yesterday.
昔、ここは公園だった。
This used to be a park.
雨だったから、行きませんでした。
It was raining, so I didn't go.