The FUNCTION keyword introduces a function definition (DEFINE) in a query (also known as query function).
The syntax after FUNCTION defines a function, which can be consumed in any other expression of the same query.
A query function overrides a model function with the same name. However, its definition is only used in other query measures and functions; other model measures and functions continue to use the model measure definition.
The following example shows using a query function in a DAX query.

DEFINE
FUNCTION SumTwoNumbers = ( 
    a : INT64 VAL, 
    b : INT64 VAL
) => a + b

EVALUATE
{
    SumTwoNumbers( 3, 2 )
}

-- Result is 5