That is used to point to a specific thing farther away in space, time, or thought, as in 'that book over there'. It also introduces extra information or clauses, as in 'I think that you’re right.'
From Old English *þæt*, used as a demonstrative word meaning 'that', 'the', or 'which'. It is related to many other little function words in Germanic languages.
Tiny words like 'that' quietly do huge jobs: pointing, connecting, and explaining. You can often delete 'that' in sentences ('I think you’re right')—your brain still knows it’s there, doing invisible grammar work.
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