A license is the means by which you grant someone else the permission to do something with your photograph without giving up copyright.
Licenses may be very restrictive, defining specific, limited uses, or they can be broad grants. Specific licenses may be drafted by lawyers on a case-by-case basis, or you can make use of preexisting templates to accomplish your licensing goals.
Note that obtaining licensed photography or images for design work is not on topic for the site; this tag is for questions from the side of the content producer — the photographer.
