It is commonly used to show the migration data and how it is distributed geographically.

The amount of migration in a particular flow line is represented by its size: the more thick the line is, the stronger the migration wave it stands for. Flow lines are drawn from a point of origin and branch out with arrows used to show the direction of movements. 

Flow map famous example

The best-known example of a flow map is Minard’s map of Napoleon’s disastrous Russian campaign of 1812.