The scala command starts the interactive shell for you, where you can interpret Scala expressions interactively:
> scala
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_121).
Type in expressions for evaluation. Or try :help.
scala>
scala> object HelloWorld {
| def main(args: Array[String]){
| println("Hello, world!")
| }
| }
defined object HelloWorld
scala> HelloWorld.main(Array())
Hello, world!
scala>
The shortcut :q stands for the internal shell command :quit, used to exit the interpreter.