It acts similiarly to Excel's SUBSTITUTE function. If you want to substitute the characters by certain characters you should use this function.
Usage
SUBSTITUTE(text, old_text, new_text)
Arguments
text
Where should it substitute the characters
old_text
Which text should it substitute
new_text
What should it substitute with.
Value
In this example we take text "CRAN", we take the old text "RAN" and replace it with "out" which in return gives us "Cout". Function will always return character class.