What is the outpouring of the Holy Spirit in Acts 2 referring to?

Pentecost (in Acts 2) was a precursor to the fulfillment of the New Covenant which will happen at Jesus second advent to fulfil the verses below,

Joel 2:28 And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions: 29 And also upon the servants and upon the handmaids in those days will I pour out my spirit.

Acts 2:15 For these are not drunken, as ye suppose, seeing it is but the third hour of the day. 16 But this is that which was spoken by the prophet Joel; 17 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: 18 And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:

What is the New Covenant for Israel?

The pouring out of the Spirit of God is one the blessings of the New Covenant promise. According to the verses below, the New Covenant blessings that Israel will receive when their Messiah returns are,

1. The pouring out of the Holy Spirit upon them.
2. The forgiveness of sins.

Jer.31:31 Behold, the days come, saith the Lord, that I will make a new covenant with the house of Israel, and with the house of Judah: ... 33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the Lord, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. 34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the Lord: for they shall all know me, from the least of them unto the greatest of them, saith the Lord: for I will forgive their iniquity, and I will remember their sin no more.

Ezek.36:24 For I will take you from among the heathen, and gather you out of all countries, and will bring you into your own land. 25 Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you. 26 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh. 27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them. 28 And ye shall dwell in the land that I gave to your fathers; and ye shall be my people, and I will be your God.

Why did this happen in ACTS?

The promise of the Kingdom was still available to Israel in Acts 2. The Holy Spirit was poured out upon the 120 believers in the upper room so that they could be equipped and empowered to proclaim the imminency of the Kingdom. The Lord had already ascended but was prepared to come again IF ISRAEL WOULD BELIEVE!!! Yet, even with the power of the Spirit to prove their message, Israel rejected the gospel and rejected Jesus and finally rejected the Spirit of God. This is when a transition took place; where God blinded the Jews (Rom.11:25) and raised up Paul to take a new grace message to the Gentiles, (Romans 11:13; Galatians 2:8; 1 Timothy 2:7; 2 Timothy 1:11). The salvation of Israel was put on hold and through their fall, salvation came to the Gentiles (Romans 11:11).

When the dispensation of Grace ends at the rapture of the Body of Christ, the Kingdom program will be reactivated and by the end of the 7 years of tribulation, a 1/3rd remnant of Israel (who fled into the wilderness, Zechariah 13:8; Matthew 24:16-20) will be saved and they will call upon their Messiah in faith. At the appearing of Christ, He will pour out His Spirit upon them (as it happened 2000-years before at Pentecost), and they will be empowered to become a royal priesthood and a holy nation (Exo.19:6) and will fulfill their ‘great commission’ to take the Word of the Lord out into all the nations during the Millennial Kingdom (Mark 16:15).

In summary

The outpouring of the Holy Spirit in Acts 2 was the shadow event of the real thing when it happens at the end of the 7-year tribulation. There is a purpose for all things in the Bible. God is on track with His great redemption plan to cleanse His creation and bring both heaven and earth into full restoration and glory under the reign of Jesus Christ (Colossians 1:16-20).