Mourning for Saul and his army

¹⁷ David took up this lament concerning Saul and his son Jonathan, ¹⁸ and he ordered that the people of Judah be taught this lament of the bow (it is written in the Book of Jashar):

¹⁹ “A gazelle^[a] lies slain on your heights, Israel.
    How the mighty have fallen!

²⁰ “Tell it not in Gath,
    proclaim it not in the streets of Ashkelon,
lest the daughters of the Philistines be glad,
    lest the daughters of the uncircumcised rejoice.

²¹ “Mountains of Gilboa,
    may you have neither dew nor rain,
    may no showers fall on your terraced fields.^[b]
For there the shield of the mighty was despised,
    the shield of Saul—no longer rubbed with oil.

 lament means a passionate expression of grief. The king and his army died, the whole Israel mourns for them! This song was created in remembrance of Saul and Jonathan. Gazelle here represents the powerful, ie Saul and his son and his men. 

In the Message version, its clearer:

Oh, oh, Gazelles of Israel, struck down on your hills,
    the mighty warriors—fallen, fallen!
Don’t announce it in the city of Gath,
    don’t post the news in the streets of Ashkelon.
Don’t give those coarse Philistine girls
    one more excuse for a drunken party!
No more dew or rain for you, hills of Gilboa,
    and not a drop from springs and wells,
For there the warriors’ shields were dragged through the mud,
    Saul’s shield left there to rot.

Dont announce the event for it is not a time of celebration, but a time of grief. SO that everyone may weep and grief and that our enemies find one less reason to rejoice. David continues on to curse the hills on which Saul and his army fell, saying that no rain or dew shall be there as Saul left the world.