It is a reference to a physical table. Aliases are created in the physical layer of the repository and are an important part of designing a physical layer. An alias table is a physical table that references a different physical table as its source (called the original table).
A Logic Table Source (LTS) have some properties and column definitions associated with it. A LTS shows how the logical objects in the logical layer are mapped to the physical layer and can be mapped to the physical tables, stored procedures and others select statements. Creating an alias creates a copy of the metadata table object that will be referenced in SQL with a new alias name. The alias name in SQL, as it is for all tables, will be derived its metadata ID. So an alias table can be a reference to any of the LTS types.
Alias tables can be an important part of designing a Physical layer because they enable you to reuse an existing table more than once, without having to import it several times.