Source database is the database you already have and want copied. Full Convert will only read it and it will not be modified in any way. You can safely use your production database as a source database for conversion.
First thing to do is to select your source database type from the list of supported database engines. If you're not sure what you have, contact support and we will help you out.
Once you select your database engine, screen changes to let you specify connection information needed to connect to the database and read it. Depending on database type, that may mean selecting folder containing database files, or a server name, port, credentials and more.
Once you fill-in connection properties, click Next to continue to Target connection screen.
Detailed information on connecting to databases can be read here: