What is security testing? Where should I start from to learn security testing.?I asked many times about it in interview as QA

I'm a Software Quality Assurance Engineer and worked on various automated tools like jmeter, appium, selenium but in multiple interviews they asked about security testing. Have you worked on security testing. I always assumed that it's a job of Ethical Hacker's. So If it's neccessary for a QA engieer to then where should I start learning. What are the best tools for security testing. Please need some serious recomendations.