A context-free grammar consists of a set of phrase types and, for each type, a set of rules for how to generate phrases of that type in terms of words and other types of phrases.  For example, a salutation (a greeting) is a kind of phrase.  In Step, the rule:

# Try: [Hello]
Hello: Hello world!

effectively says that a “Hello world!” is a kind of Hello phrase.  Technically, the part before the colon tells us that we’re defining a little program called Hello and when we run it, it should print “Hello world!” Click on the box above, press the Run button, and you should see something like this:

Hello world!

Theme and variation

We can define several different salutations:

Hello: Hello world!
Hello: Hola!
Hello: Hi there!
Hello: Bon jour!

Each of these lines specifies a different way of saying hello.  For reasons that become clear soon, if two rules will both work, Step will prefer the one that appears first in the list.  But we can override this by adding the annotation [randomly] at the beginning, which tells it to choose the rules randomly:

# Try: [Hello]
[randomly]
Hello: Hello world!
Hello: Hola!
Hello: Hi there!
Hello: Bon jour!

Again, try clicking the code above and pressing Run.  But press it several times.  You should get different results.