Statuses allow you to group leads by their standing or track where they are in your process. The default status is the status given to leads as they are created through importing or a form submission. Therefore, the status you have set as "Default" will be the status they receive.

To change your default status, follow the steps outlined here. 

Summary

Statuses allow you to group leads by their standing or track where they are in your process. The default status is the status given to leads as they are created through importing or a form submission. Therefore, the status you have set as "Default" will be the status they receive.