Business may be understood as the organized efforts of enterprise to supply consumers with goods and services for a profit. Businesses vary in size, as measured by the number of employees or by sales volume. But, all businesses share the same purpose: to earn profits.
Usually, when filling out some kind of form, "nature of business" refers to the type or general category of business or commerce you are describing. For example, if you worked at McDonald's, the nature of your business is food services.