From Teflpedia

That (/ˈðæt/) is an English word that can act as either a conjunction, a relative pronoun or a singular distant demonstrative.[1]

As a conjunction, it can be used in various ways. Its weak form is /ðət/

Often it can be dropped, compare "I think that Trump is awful" v. "I think Trump is awful".

As a determiner, it has the plural form those, and the proximate equivalent this.