Hello!
The equator splits the Earth into two portions, which are known as hemispheres. The section of Earth above the equator is known as the Northern Hemisphere. The section below is known as the Southern Hemisphere.
Meridians are simply lines of longitude. However, Earth is also divided into two vertical portions, known as the Eastern and Western Hemispheres. The Prime Meridian is the vertical line that splits Earth into these two sections. The Prime Meridian is also sometimes referred to as the "Greenwhich Meridian" because it runs through Greenwhich in the UK.
I hope this helps answer your question! Have a great day!