In order to generate a flat map, features on the spherical
Earth are projected onto a surface. To create this map,
the shapes of the continents, along with latitude and
longitude lines, were projected onto a cylinder. Then
the cylinder was split and flattened. This type of projection
is called a Mercator projection.
You can visualize the process of "projection"
for this map by imagining a light shining at the center
of the sphere, and considering the cylinder around it
as a screen. Lines and shapes projected onto the screen
by the light become the flat map.