Independent requirements and dependent requirements are the two main types of requirements you encounter when configuring SAP planning. This is what they are:
Planned Independent Requirements (PIR)
Planned Independent Requirements are planned production or sales quantities that are based on some sort of forecast procedure (e.g. Material Forecast or S&OP procedure). These numbers are used in MRP to calculate procurement and/or production quantities for a material.
(Planned) dependent requirements
Dependent requirements is demand that is dependent on another material. E.g. components of a bill of material are often planned on the basis of the demand for the material of which it is a part.
You plan to sell 200 bikes in the month of January 2009 based on sales in the same month of 2008. These 200 bikes is a Planned Independent Requirement for 01/2009.
Since every bike is produced using two pedals the 200 bikes you plan to sell in January 2009 results in 400 pedals you need to procure. These 400 pedals is a planned dependent requirement for 01/2009.
On the basis of these numbers MRP can calculate on the basis of stock levels how much bikes and pedals you actually need to produce/procure.