God will send God's Holy Spirit to help David’s family and the people in Jerusalem pray. They will look at the one they hurt. They will cry hard, like for a first son.
Then I will pour out on the house of David and on the people of Jerusalem a spirit of grace and prayer, and they will look on Me, the One they have pierced. They will mourn for Him as one mourns for an only child, and grieve bitterly for Him as one grieves for a firstborn son.
KJV
And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for [his] only [son], and shall be in bitterness for him, as one that is in bitterness for [his] firstborn.
ASV
And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplication; and they shall look unto me whom they have pierced; and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his first-born.