In early February, immediately following his State of the Union address, the president submits his budget proposal to Congress, where two budget committees -- one from the House of Representatives and one from the Senate -- debate, amend, and vote on its overall size and on the distribution of its spending among various accounts. The House and Senate each have different rules governing how closely to follow the guidance of the president's budget proposal, and the members of the budget committees choose which of its themes and suggestions to adopt and incorporate into the congressional budget resolution they now formulate.
According to the official rules of the process, the two budget committees are to resolve any differences at this stage so that the same budget resolution can be presented to both the House and Senate for consideration. In reality, it is not absolutely necessary for the House and Senate to agree on a single version at this stage. Ultimately, what matters is that the two chambers of Congress eventually agree on budget allocations at the agency and program level later in the process.