This verse is a stark and devastating summary of a pivotal battle between the Israelites and the Philistines. Here's a breakdown of what it means:

"Now the Philistines fought against Israel...": This simply states that a military conflict was taking place between the Philistines and the Israelites. These two groups were long-time rivals, vying for control of the region.

"...and the men of Israel fled from before the Philistines...": This indicates that the battle was not going well for the Israelites. They were being defeated and were retreating. Fleeing in battle was often a sign of disarray, panic, and ultimately, a loss.

"...and fell down slain on Mount Gilboa.": This is the most tragic part of the verse. It means that many Israelite soldiers were killed (slain) during the battle and the retreat, and they died on Mount Gilboa. Mount Gilboa was the location of the battle and a geographically important high ground.

In essence, this verse is telling us that the Israelites suffered a crushing defeat at the hands of the Philistines on Mount Gilboa. It sets the stage for the rest of the chapter, which details the deaths of King Saul and his sons, including Jonathan.

The verse is significant for several reasons:

It marks a major turning point in Israelite history. The defeat was a severe blow to Israel's military and political power.
It foreshadows the tragic fate of King Saul and his family. The verse immediately precedes the account of their deaths, emphasizing the consequences of the defeat.
It highlights the ongoing struggle between Israel and the Philistines. This conflict was a recurring theme in the Old Testament and shaped the history of the region.
It conveys the grim reality of war. The verse is direct and unflinching in its description of death and defeat.