Portfolio theory

This seems a fairly open question. It is basically a technique of reducing risk by investing carefully in a number of different companies and sectors. In this way unsystemic risk can be reduced or even effectively eliminated but systemic (also caused market risk) can never be eliminated. See following for more details-

https://www.accaglobal.com/pk/en/student/exam-support-resources/fundamentals-exams-study-resources/f9/technical-articles/capm-part1.html

Hope this helps. Please ask if you want more information.