What does system landscape for SAP means?

Dear experts,

I know the system landscape in SAP (through Menu Path: TCode: STMS---> Transport Routes) is used to validate if the development, test, and production environments are properly segregated in the system. However, I want to know the meaning of the following symbols and flow, as depicted in the illustration below. I am a newbie to SAP, and I want to know in layman's term the technicalities presented in the landscape.

My specific questions are:

1. What does DEH and ZDEH, QE1, and PE1 means?

2. What does Transport Lay and Delivery means?

3. What is the meaning of the arrows/flows interconnecting the boxes?

Thank you very much for all the insights.

Regards,

Frances