Batch input is used to transfer large amounts of data into the SAP system. When transferring data into the SAP system, we can refer to two types of transfers:
Conversions - This type of transfer refers to a one-time transfer from a legacy system to the SAP system. In this case, the “legacy” system is the old system that is being replaced by the SAP system. For example, the old system being replaced has data for 2,000 vendors that must be transferred to the SAP system.
Interfaces - This type of transfer refers to an ongoing transfer from a complementary system to the SAP system.  In this case, the “complementary” system is a system that will run along side the SAP system. For example, customer orders may be taken in another system.  For the SAP system to reflect accurate information, these orders must be transferred to the SAP system every night.
Batch input is also referred to as BDC (Batch Data Communication).

Online Program

Data passes through checks and validations when a user enters the data online through a transaction (i.e., an online program). Automatic checks are performed on each screen of an online program: Required fields, Proper formats, Foreign key values, Valid values etc.
The checks programmed into the online program are also performed on the data as the transaction progresses.
To ensure that data passes through these checks during batch input, you must simulate the user dialogue by passing the data through an online program.

BDCDATA Structure
To simulate user dialogue (i.e., to pass data through an online program), you must know the following information: Online program name, Screen numbers, Field names and Field values
When you code a batch input program, the ABAP Dictionary structure “BDCDATA” is used to collect this information.  The “BDCDATA” structure includes the following fields:
PROGRAM [CHAR 40] - Online program name.
DYNPRO [NUMC 4] - Screen number.
DYNBEGIN [CHAR 1] - Flag to indicate the start of a new screen.
FNAM [CHAR 132] - Field name of a screen field to be filled with data.
FVAL [CHAR 132] - The actual value to be filled into the specified screen field.
To collect this information for an entire transaction, the “BDCDATA” structure will be used to create an internal table in the batch input program.