These words are what STEP Bible calls "demonstrative pronouns".
This is an uncommon phrase in English, which makes it a little hard to understand. Basically,
{demonstrative pronouns} = {the words for this and that"}
There are several demonstrative pronouns in Greek...but 90% of the demonstrative pronouns in the New Testament are forms of either οὗτος or ἐκεῖνος. If we learn these, it will help us again and again in our translation work. Together, they occur around 1,400 times.
By now, some of these word endings are starting to look pretty familiar to you.
Decl. 2 1 2 Gender M F N Nom. S. οὗτος αὕτη τοῦτο Gen. S. τούτου ταύτης τούτου Dat. S. τούτῳ ταύτῃ τούτῳ Acc. S. τοῦτον ταύτην τοῦτο Nom. P. οὗτοι αὗται ταῦτα Gen. P. τούτων τούτων τούτων Dat. P. τούτοις ταύταις τούτοις Acc. P. τούτους ταύτας ταῦτα
Decl. 2 1 2 Gender M F N Nom. S. ἐκεῖνος ἐκείνη ἐκεῖνο Gen. S. ἐκείνου ἐκείνης ἐκείνου Dat. S. ἐκείνῳ ἐκείνῃ ἐκείνῳ Acc. S. ἐκεῖνον ἐκείνην ἐκεῖνο Nom. P. ἐκεῖνοι ἐκεῖναι ἐκεῖνα Gen. P. ἐκείνων ἐκείνων ἐκείνων Dat. P. ἐκείνοις ἐκείναις ἐκείνοις Acc. P. ἐκεῖνους ἐκείνας ἐκείνα
οὗτος and ἐκεῖνος can function like pronouns. In which case, they translate as "this" or "that", "these" or "those", standing in for a noun.
Matt. 3:3 οὗτος γάρ ἐστιν ὁ ῥηθεὶς διὰ Ἠσαΐου τοῦ προφήτου λέγοντος
For this is the one about whom Isaiah the prophet had spoken.
They can also function like an adjective, describing the noun they are attached to.
ὁ ἄνθρωπος οὗτος
This man.
Sometimes these words overlap in function with the definite article or αὐτός.
Lk. 1:32 οὗτος ἔσται μέγας καὶ υἱὸς Ὑψίστου κληθήσεται
He will be great, and will be called the Son of the Most High