What does Horizontal Control mean in GTD?
It refers to the coherent management of all the activities in which you are involved in your day-to-day life. Basically, it is the administration of all your ongoing actions and projects.
It is about clearly defining the outcomes you need to achieve and the next actions required to achieve them, and creating reminders in a trusted system that you review regularly. Therefore, you need a system that allows you to track all those reminders and supply required information about them on demand, so that you can do what really makes sense at all times.
For this horizontal control to make sense you will need to also lean on a higher perspective, also called Vertical Control.