Building API proxy

[#1]

While building the API proxy, under security section: i got like this

Secure Access for User and Clients:

Authorization:

• Pass Through(None)
  
• API Key
  
  • Impose quota per developer
    
  • Publish API Product

Browser:

• Add CORS Header

I am new to APIGEE API Gateway, it would be helpful if someone help me to understand the purpose of using those.