Certainly. It's always a pleasure to explain these fiscal and budgetary issues.
The main estimates and the supplementary estimates are the way by which the government finances its operations. Because the government cannot spend money that has not been approved by Parliament, it has to seek approval, either through stand-alone legislation—such as, for example, through the Old Age Security Act to spend in an ongoing manner for old age security, as one example—or it can seek approval for an annual spending authority, such as for the functioning of Parliament, of government operations and departments, and so on. There needs to be legislation to allow the government to spend. That is done, usually, through the main estimates or the supplementary estimates.
The budget, on the other hand, is a document that lays out policy priorities and the way the government will be exercising that authority and implementing those priorities, and that includes changes to tax policy and changes to policy that may not have any expenditure implications. Those exercises are not identical, but they are broadly similar.
It's not a very good explanation, I know, in a short period of time.