Are these production rules for a formal grammar?
The use of notation in that statement might be confusing.  A grammar is a rule of the form &quot;something $\to$ something&quot;, where the something on the left is a string of symbols containing at least one non-terminal symbol, and the something on the right is a string of symbols.
For instance, $A \to BcD$ would be a valid rule, as would $AB \to BDeeF$, or $aAcE \to b$.
If you read the very next sentence after the one you quoted, it explains the meaning of the part you didn't understand, in different words.  It says &quot;That is, [...]&quot; (you can read the rest yourself).  If you don't understand one sentence, in the future I suggest you keep reading a little further to see if you can work it out from context.