A Signed Mark Data (SMD) file is similar to a password in the sense that it allows you to identify yourself and request an action that requires privileges to be executed.
The SMD file is a versatile instrument that will allow you to register domain names during the sunrise period of new gTLDs and request other services (e.g. initiate an URS process).
Note: The possession of your SMD is sufficient to request services based on your SMD, therefore it is important that you keep your SMD secure and you only share your SMD with a trusted person/entity (e.g. ICANN-approved URS providers).