REPL is a tool for evaluating expressions in Scala. It can be started from the command line by entering scala or directly from the SBT session. The nice thing about starting it from the SBT shell is that all of the dependencies configured for the project will be available, as shown in the next screenshot:

Please refer to the Scala documentation at https://docs.scala-lang.org/overviews/repl/overview.html to learn how to use REPL efficiently.

To exit REPL and return to the SBT shell, enter :q.