The Haftara describes the collection of funds which was made during the time period of King Yoash in order to repair and rennovate the Bet Ha'Mikdash.
To understand why it was necessary to rennovate the Bet HaMikdash during the time of Yoash, we must review the events that led to his kingdom.
Recall from last week's Haftara that Achav, the king of Israel, made a treaty with the King of Sidon which led to the proliferation and legitimization of Baal worship in Israel.
At the same time that Achav was king of Israel, Yehoshafat was king of Yehuda. Although Yehoshafat was a good king, he had also entered into a military treaty with Achav. This friendship ultimately led to the marriage of Yeshofat's son Yoram with Achav's daughter Atalya. Due to the bad influence of the house of Achav, both Yoram and his son and succesor Achazya became bad kings (see II Melachim 8:16-29).
Achazya was killed during Yehu's revolt against the house of Achav (see II Melachim chapters 9-10). In retalition for Yehu's murder of the members of both royal families, Atalya, the only remnant of the House of Achav, murdered the entire royal family of Yehuda. Only Achazya's son Yoash, who was only an infant at that time, was saved (see 11:1-2). Yoash was secrectly kept, while Atalya took over the kingdom of Yehuda.
Therefore, from the time of Yehoshafat's death, i.e. during the reign of Yoram, Achazya and Atalya, the Bet HaMikdash in Yerushalayim became quite neglected.
After six years in power, Atalya was finally ousted by a coup'de'ta orchestrated by the High Priest, Yehoyada. Yoash, then only seven years old, was proclaimed king. During the early years of his reign, Yoash was a good king. With the guidance of Yehoyada, proper worship was restored in the Bet HaMikdash. Nonethelees, the Mikdash itself required major rennovations.
Finally, in the 23rd year of his reign, Yoash, together with Yehodaya, made a special effort of raise funds for proper fixing of the Mikdash. Towards that purpose, Yehoyada makes the first "pushka" (tzdaka box - see 12:7-12), and places it in the Bet HaMikdash near the mizbayach. From the funds collected, they were able to pay wages of workers who finally finished the rennovation of the Temple (12:13-16).