regular google search ?q=what is a Security?

"Security" a thing deposited or pledged as a guarantee of the fulfillment of an undertaking or the repayment of a loan, to be forfeited in case of default.

Investopedia

The term "security" refers to a multitude of different investments, such as stocks, bonds, investment contracts, notes, and derivatives. For example, a security can represent ownership in a corporation in the form of stock, a creditor relationship with a governmental body or corporation in the form of a bond, or rights to ownership in the form of an option. [1] Security

Use this section to tell people about which versions of your project are currently being supported with security updates.

Use this section to tell people how to report a vulnerability.

Tell them where to go, how often they can expect to get an update on a reported vulnerability, what to expect if the vulnerability is accepted or declined, etc.