When a user decides to merge multiple records (either through a Review Group or using Mass Merge) rules control how the merge is performed.  There are two types of rules available:

Master Record Rule - these rules determine how the master (or surviving) record will be selected.

Field Value Rule - these rules determine how fields will be merged.  By default, Salesforce merge retains only the data in the Master record.  This feature will allow users to customize the merge process to retain other data.

Learn more
Manage Merge Rules
Merge Rules: Create Master Record Selection
Merge Rules: Creating Field Value