A data layout defines the structure of data for an endpoint. Think about data layout as the XML schema (xsd). Each data layout can have one or more data fields which are then mapped to its counterpart (source to destination).
Predefined (added from Wizard) systems come with endpoints and their data layouts defined however manually added systems have blank layouts and you must define it manually. There are tools which can help you do so such as the editor where you paste a samle JSON/XML or file upload where you can drop a sample file from which DSYNC generates a data layout for you. You should always check that the data layout and all its fields are correct after using these tools. A field must have a correct type, you should add a 'required' parameter where required and check the data layout structure is as per your source/destination data.