The substring function takes 3 arguments. The first argument is the actual string in which you want to extract from. The second argument is the start position, and the third argument is the number of characters to extract.
SET @BaseString = 'Quick Brown Fox'
@BaseString -- The base string to extract from
,7 -- Start Position
,5 -- Length of Characters
The above query returns the value ‘Brown’.
Note: This is equivalent to the Oracle function: substr