大体（だいたい） is an adverb that is used to highlight the ‘main point’ of something in Japanese, and therefore has several different nuances.
大体（だいたい） will regularly be translated as ‘mostly’, ‘generally’, or ‘for the most part’.
As with other adverbs that highlight amounts,
大体（だいたい） can be used to modify entire phrases, or can be used directly before a noun as
大体（だいたい）の + noun.
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だいたいでいいから何時（なんじ）くらいに着（つ）くか教（おし）えて。
Since a general time is okay, can you let me know around when you will arrive?
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水曜日（すいようび）はだいたい5時（じ）に起（お）きています。
On Wednesdays, I generally wake up at 5 am.
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だいたいの人（ひと）たち薔薇（ばら）という漢字（かんじ）を書（か）けません。
Most people do not know how to write the kanji ‘bara’. (薔薇（ばら） is the kanji for ‘rose’)
In a similar way to そもそも, a noun/adverb which means ‘in the first place’,
大体（だいたい） can also have this meaning. This nuance is frequently quite obvious, as the rest of the sentence will highlight a desired outcome that unfortunately did not happen.
Although these 2 primary uses of
大体（だいたい） may seem quite different, they are both just expressing what is ‘essentially’ correct, most common, the original, etc.
A common synonym for
大体（だいたい） is 根本（ねもと）, a noun which means ‘the source’, or ‘the origin’ or something. In this way,
大体（だいたい） just expresses the way something is, without receiving any alterations (regardless of whether any subsequent alterations are positive or negative). This is where the ‘in essence’ meaning comes from.
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と言（い）うことは私（わたし）たちが言（い）っていることはだいたい一緒（いっしょ）なのね。
In that case, what we are both saying is the same in essence.