The weaponization step in the Cyber Kill Chain Model involves the creation or use of a specific weapon (malware, exploit) designed to leverage a vulnerability.
This phase follows the reconnaissance phase where the attacker gathers information and precedes the delivery phase where the weapon is delivered to the target.
Developing specific malware to exploit a vulnerable server is a precise example of weaponization.
References
Lockheed Martin Cyber Kill Chain Model
Understanding the Weaponization Phase in Cyber Attacks
Steps in the Cyber Kill Chain