To get the most out of the migrate module, one should understand the following concepts:

Content set

This represents a set of source content (embodied in a view) to be migrated. It includes mappings from columns in the view to fields within Drupal objects.

Destination

A Drupal object designated to receive migrated data from a source view. Most destinations are simple - roles, terms, users - but nodes are more complex - each distinct node type defined in your Drupal site is a distinct destination in the migrate module.

Clear

An operation which deletes previously-migrated content, generally in preparation for remigration.

Import

An operation which imports content represented by a content set into Drupal objects. By default this operation only processes source data which has not yet been migrated, but optionally it can also update previously-imported destination objects with the current source data.

Map tables

A table associated with each content set, holding mappings from the unique ID of a source view row to the unique ID of the Drupal object created from that row. This is used to keep track of which rows have been processed, and can also be used to develop audit views directly comparing source data to resulting Drupal data.

Primary key

In relational databases, a table's primary key consists of one or more columns which are unique for each row and never null, explicitly designated to uniquely identify each row. The migrate module uses this term to describe a unique non-null column for a source view - in most cases, this will actually be the primary key of the base table of the view, but you may designate another unique column to function as a primary key for the purpose of migration. This column is used as the source ID in map tables.