If your students are using the Riggs program, they already know that spoken words are the signs of ideas. They also know that written words are symbols for spoken words.
If you have gotten past lesson 36, your students also know that words can be strung together to form sentences, which are the signs of thoughts.
A phonogram is a combination of phoneme and grapheme. In Greek, "phone" means sound; "gram" means a written letter. Each grapheme stands for one phoneme in a given word.