What is Microsoft office security essentials and how does it work?

Dear Karmel it is Microsoft Security Essentials (MSE) not Microsoft Office Security Essentials, Microsoft Security Essentials is an antivirus software product by Microsoft it doesn't need subscription and from recent OS it's an pre built application that provides protection against different types of malicious software such as computer viruses, spyware, rootkits and Trojan horses.

Just for your brief understanding competitor products are Kaspersky, Symantec, McAfee, AVG etc.