• 約束やくそく — promise / agreement
• 結束けっそく — unity / solidarity
• 花束はなたば — bouquet
• 札束さつたば — wad of bills
• 束縛そくばく — restraint / shackles

The character shows items bound tightly together, symbolizing control or unity. With the 木 (tree) radical, it also suggests wood or sticks being tied in a bundle. By extension, it came to mean promises, agreements, and unification.