What does 1 Chronicles 12:37 mean?

From the east side of the Jordan, the tribes of Reuben, Gad, and the half-tribe of Manasseh sent 120,000 men, armed with every kind of weapon. 1 Chronicles 12:37 - Modern Text Bible

(The tribes living east of the Jordan sent a huge, well-armed force, showing that support for David was truly nationwide.)