What are the Gray Characters in Intellij


Using intellij idea community for scala development. I can see some characters are added into the code by my editor, that are there, but not there, as in I cannot edit or remove them. The typing cursor just moves past them, look at this screenshot, the index=. 

I have never witnessed such a thing in any IDE. What does this mean. Are those characters inserted by the IDE? 


You can read more about inlay hints here.
Also there are some Scala-specific hints, like "Implicit hints".
You can read more about them here.

in the picture , those gray characters are called hint parameters,  they are inserted by the ide ,you can hide them by this way 

File -> Settings -> Editor -> Inlay Hints