what does include do in create index sql server code example

Example: create index sql server syntax

-- Create a nonclustered index on a table or view
CREATE INDEX i1 ON t1 (col1);

-- Create a clustered index on a table and use a 3-part name for the table
CREATE CLUSTERED INDEX i1 ON d1.s1.t1 (col1);

-- Syntax for SQL Server and Azure SQL Database
-- Create a nonclustered index with a unique constraint
-- on 3 columns and specify the sort order for each column
CREATE UNIQUE INDEX i1 ON t1 (col1 DESC, col2 ASC, col3 DESC);