The first part sets up the scenario as a dialogue:
1 ἵνα τί ἐφρύαξαν ἔθνη καὶ λαοὶ ἐμελέτησαν κενάThen comes the speech attributed to the kings and leaders:
2 παρέστησαν οἱ βασιλεῖς τῆς γῆς καὶ οἱ ἄρχοντες συνήχθησαν ἐπὶ τὸ αὐτὸ κατὰ τοῦ κυρίου καὶ κατὰ τοῦ χριστοῦ αὐτοῦ
1 Why do the nations behave wantonly, and the people strive for that which is empty?
2 On account of this the kings of the earth and the leaders gather together against the Lord and against his Christ.
The bonds of the Lord and his Christ are not something that these leaders want to submit themselves to.
3 διαρρήξωμεν τοὺς δεσμοὺς αὐτῶν καὶ ἀπορρίψωμεν ἀφ' ἡμῶν τὸν ζυγὸν αὐτῶν
3 We will rip open their bonds and cast away their yoke from us.
The speech of the Lord is set up as rejecting the leader's argument in anger.
4 ὁ κατοικῶν ἐν οὐρανοῖς ἐκγελάσεται αὐτούς καὶ ὁ κύριος ἐκμυκτηριεῖ αὐτούς
5 τότε λαλήσει πρὸς αὐτοὺς ἐν ὀργῇ αὐτοῦ καὶ ἐν τῷ θυμῷ αὐτοῦ ταράξει αὐτούς
4 He who sits in the heavens will laugh at them and the Lord will mock them.
5 Then will he say to them in his wrath and trouble them in his anger.
Then comes the conclusion.
6 ἐγὼ δὲ κατεστάθην βασιλεὺς ὑπ' αὐτοῦ ἐπὶ σιων ὄρος τὸ ἅγιον αὐτοῦ
7 διαγγέλλων τὸ πρόσταγμα κυρίου κύριος εἶπεν πρός με υἱός μου εἶ σύ ἐγὼ σήμερον γεγέννηκά σε
8 αἴτησαι παρ' ἐμοῦ καὶ δώσω σοι ἔθνη τὴν κληρονομίαν σου καὶ τὴν κατάσχεσίν σου τὰ πέρατα τῆς γῆς
9 ποιμανεῖς αὐτοὺς ἐν ῥάβδῳ σιδηρᾷ ὡς σκεῦος κεραμέως συντρίψεις αὐτούς
6 I set up the king under him on Zion, his holy mountain,
7 promulgating the decree of the Lord. The Lord said to me: Thou art my son; today I have begotten thee.
8 Ask me and I will give thee the nations for thy inheritance and the ends of the earth as thy possession.
9 Herd them with an iron rod; as a potter's vessel smash them.
In the Septuagint, this psalm explicitly identifies the Christ with the Son of God. (The Christians did not invent that idea.)
10 καὶ νῦν βασιλεῖς σύνετε παιδεύθητε πάντες οἱ κρίνοντες τὴν γῆν
11 δουλεύσατε τῷ κυρίῳ ἐν φόβῳ καὶ ἀγαλλιᾶσθε αὐτῷ ἐν τρόμῳ
12 δράξασθε παιδείας μήποτε ὀργισθῇ κύριος καὶ ἀπολεῖσθε ἐξ ὁδοῦ δικαίας ὅταν ἐκκαυθῇ ἐν τάχει ὁ θυμὸς αὐτοῦ μακάριοι πάντες οἱ πεποιθότες ἐπ' αὐτῷ
10 and now, O kings, understand; learn all who judge the earth.
11 Serve the Lord in fear and rejoice in him with trembling.
12 Serve the Son lest the Lord be angry and you will be destroyed from the way of righteousness when his anger is quickly kindled; blessed are all who trust in him.
One of the things that intrigues me about this psalm is the imagery and promises are very similar to the promises that the gods make to the Egyptian kings. In the Egyptian temples, the Egyptian gods often promise the Egyptian king that he will rule over everything that the sun encircles. Other nations are promised as the king's inheritance. The king is designated as the son of the god. The gods are to be feared and reverenced or they will be angry and burn their foes. Smashing pottery vessels as a means of cursing enemies is actually a ritual for the ancient Egyptians.
So this particular psalm has a clear ancient Near Eastern background, but since it was quoted half a dozen times in the New Testament, it was seen as a particularly Christian psalm.