A license allows a content supplier to define the conditions under which they will license access to their content such as blog entries, uploaded files etc. The license conditions are set by the supplier (the license issuer) and when another user wishes to access the content covered by a license, they must accept the terms and conditions of the license. This feature is available to all members.
Licenses can do a number of things such as restricting or granting rights to the customers to use the content in certain ways. They also allow the supplier to define a fee that customers must pay and the terms of the payment, i.e. single one off or subscription based payments.
A full set of Licensing tutorials can be found here