If you the connection with the source database is not established then you will get below message:
Go back to options and correct the database credentials.
Just choose what you want to transfer and press button transfer. If the connection is healthy then the 'Transfer' button will become enabled and below screen we appear:
You have the option to transfer all items of what you choose, or selected items based on their ids. You may choose individual ids, or range, or a combination of both. For example:
Will transfer items with ids: 3 and 68, and range of id's between 10 to 56, and range 100 to 145. If you leave empty the ids field then all items will be transferred.
Also you may press button Clear to clean corresponding field, or Choose to visually choose individual items. The choose button will open a well known window depended on the type of items to choose with search and filtering capabilities. See below of example of choosing articles. Similar dialogues exist for all items.
Choosing ids could be helpful if you do not wish to bring all the data from the source site.
After the transfer of items you just choose all and leave the ids fields empty and press button finalize. This step will fix the links in case you chose to bring items with new ids.