In SQL Server 2016 what is sys.tables is_external field?
This is my interpretation ..

SQL Server 2016 allows you to configure POLYBASE - accesses and combines both non-relational and relational data - from SQL Server.

It allows you to run queries on external data in Hadoop or Azure blob storage.

SQL server has a column is_external to identify that the table is external. You can find more on external tables using sys.external_tables catalogue view.