A "glider board" is a little platform that you can attach to a stroller, allowing an older child to stand on it while you push the baby. Full definition
We prepared this list for you with the detailed explanation for each feature to help you really choose the Best Baby Jogger Glider Board. (thankyourbaby.com)
It can act as a single stroller, a tandem double stroller and even a triple, when equipped with an optional Glider Board for a third kid. (mommystroller.com)
It has a very large glider board which makes working out more comfortable for larger users. (agymrat.com)