Meaning of “that” - Learner’s Dictionary

that

conjunction uk us strong /ðæt/ weak /ðət/
AFTER VERB/NOUN/ADJECTIVE

A2 used after some verbs, nouns, and adjectives to introduce a new part of a sentence:

He said that he'd collect it later.
Is it true that she's pregnant?
RELATIVE CLAUSE

B1 used instead of 'who' or 'which' at the beginning of a relative clause:

Have you eaten all the cake that I made yesterday?

(Definition of “that conjunction” from the Cambridge Learner’s Dictionary © Cambridge University Press)