This page allows you to view and edit a content set, in particular to add mappings from source data to Drupal fields. The content set attributes are as follows (note that the source view and destination are inherent to the content set and cannot be changed after it is created):

Description
A user-friendly name for the content set, used for display in various places. It also may be used to specify which content set to process on a drush command.
Source view
A view containing the source data for this content set. This view must contain a single row for each destination object to be created (e.g., a single row per user, or per Page node). It also must have at least one unique data column. Views with exposed filters will not work with migration - they will always return 0 rows.
View arguments
It may be that you want to use a single view as the source for multiple content sets - for example, you might have an incoming "content" table which contains both articles and blog entries. In this case, you would add an argument to the view definition to distinguish the cases, and create separate content sets for the different cases. Here you specify the distinct argument used to select rows from the view for this content set.
Destination
The Drupal object which this content set will populate. The migrate module has builtin support for the following Drupal objects, and you can also use hooks to define customer destinations:
Primary key
A unique non-null column from the source view, which the migrate module will use in maintaining a map between source rows and resulting Drupal objects.
Weight
A numerical value used to order content sets in listings, and when processing multiple content sets in a single operation. For example, if you are migrating roles, then assigned migrated roles to users, you should give your role content set a lower weight than the user content set so it is processed first.

Content mappings

Here is the crux of the process - specifying what source fields map to what Drupal fields. For each destination field, you may choose a column from the the source view, an explicit default value, both, or neither. If you specify only a source field, that value will be copied to the mapped Drupal field on import. If you specify only a default value, that value will be copied. If both are specified, the value of the source field will be used if present - if it is not (the value is NULL), the default value will be used. If nothing is specified, the destination field will default according to its own rules - generally it will be left empty, but some fields may receive default values (for example, Node: Authored on will default to the current date and time).