This verse is the opening of Joshua 11 and describes the reaction of the Canaanite kings to the Israelite victories in the previous chapters. It means:

Jabin king of Hazor: Jabin was the most powerful king in northern Canaan at this time. Hazor was a major city.
heard of it: He heard about the Israelite victories, specifically the defeat of the southern coalition of kings led by Adoni-zedek of Jerusalem (Joshua 10). This victory was a major threat to the Canaanite kings.
he sent to Jobab king of Madon, to the king of Shimron, to the king of Achshaph: Jabin sent messengers to other kings in the region, specifically those of Madon, Shimron, and Achshaph. He was rallying them to form a coalition to fight against the Israelites. He was calling for a military alliance to defend themselves against the invading Israelites.

In essence, this verse sets the stage for the next major battle in the book of Joshua. Jabin, recognizing the threat the Israelites pose, is taking the lead in organizing a unified Canaanite resistance. This verse highlights the fear and urgency felt by the Canaanite kings, motivating them to band together despite their usual independence.