MGA (a.k.a. MVCC) is a feature used by many modern DBMS systems. It means that multiple versions of records are kept in the database as long as at least one transaction needs them.
Each active transaction has its view of the database which is consistent in any moment, because the records it needs to see at not deleted or changed even if other transactions modify or delete them. This enables the famous "readers don't block writers and writers don't block readers" feature.
MGA ensures that both OLTP and OLAP operations can happen simultaneously without need for locking found in legacy database systems.