Business Requirements is like the recipe for your project. You perform a process of gathering business requirements via any number of techniques; workshops, observation, meetings, questionnaires just to name a few.
You collect all the facts and figures you need about the project. These may be details of existing processes, business problems, new products or organizational restructure requirements. You need to understand the business need to ensure that you create a reliable solution that will meet that need. There are also various business analysis tools available to assist in this process of gathering and analysis.
This very important and sometimes lengthy process results in the definition of what will meet the need of the business, it is the requirement definition itself. Again depending on how you choose to operate this can be as a requirements document or recorded in a requirement management tool.
The tricky thing about this is you need to be able to relay to the customer or business themselves what you have found and determined, and you also then need to be able to translate that into the instructions that will be carried out. This may mean Technology or Human Resources for example also need to understand the requirements.