Let's define a simple grammar which allows us to write a single digit from 0 to 9.

Above is the rule for a single digit. Reading it from left to right it says

Let the rule digit be defined (or assigned) as being the non-terminal 0 or 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9.

The phrase terminal may seems complicated but all it is saying is that we can not break down a terminal symbol anymore. We shall look at non-terminals next.