The map can be accessed by the button in the top left of the screen or by the hotkey [M].
It shows you a top-down view of the land that is visible to you along with the amounts of locations of natural resources available to you. You can zoom in and out using the mouse wheel or the slider on the left.
Natural resources outside your fog of war are marked as unknown and may be any natural resource type, but their location can give you a hint about which direction to expand in.
Use the map to determine in which direction to sail your expedition boat by looking at the markers for unknown resources.