Explanation
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0 In computing, a user agent is any software, acting on behalf of a user, which "retrieves, renders and facilitates end-user interaction with Web content".[1] A user agent is therefore a special kind of software agent.
https://en.wikipedia.org/wiki/User_agent#User_agent_identification
A user agent is a computer program representing a person, for example, a browser in a Web context.
https://developer.mozilla.org/en-US/docs/Glossary/User_agent

Explanation
Delivery: This step involves transmitting the weapon to the target.
Weaponization: In this step, the intruder creates a malware weapon like a virus, worm or such in order to exploit the vulnerabilities of the target. Depending on the target and the purpose of the attacker, this malware can exploit new, undetected vulnerabilities (also known as the zero-day exploits) or it can focus on a combination of different vulnerabilities.
Reconnaissance: In this step, the attacker / intruder chooses their target. Then they conduct an in-depth research on this target to identify its vulnerabilities that can be exploited.