Governments have money from different sources in order to manage countries. They use the money for different purposes and generate investments in order to keep that money growing.
As people, countries need to expend money and use it the best way possible. The diagram shows a budget deficit because the government is expending more than what is getting so the budget is shorter than what they are needing, a deficit in the budget means that the country is facing a difficulty calculating what they need and is using an strategy that is not working,
A deficit in the budget then means that, the budget is lower than what is being needed.