We all tend to use energy at around the same times of the day, that means there are times where there is a need for lots of energy and so lots of fossil fuels get burned, and on the flip side there are times where there is more than enough renewable energy to go around. This causes problems for the national grid.
Balancing the grid is about trying to solve those problems by reducing how much fossil fuel we use but always making sure that their is enough energy to keep the lights on and homes warm.