As a demonstrative pronoun (pl. Those), that usually points out, or refers to, a person or thing previously mentioned, or supposed to be understood. That, as a demonstrative, may precede the noun to which it refers; as, that which he has said is true; those in the basket are good apples.
As an adjective, that has the same demonstrative force as the pronoun, but is followed by a noun.
As a relative pronoun, that is equivalent to who or which, serving to point out, and make definite, a person or thing spoken of, or alluded to, before, and may be either singular or plural.