IDoc stands for Intermediate Document in SAP. It is nothing but a SAP Document format that is used for Business transaction data transfers.
For us technical people, we can understand it by saying that IDoc has a purpose similar to that of XML.
Both are used for data exchange and automation in computer systems, but the IDoc-Technology takes a different approach.

An IDoc consists of -

Control record - It defines the type of IDoc, port of the partner, release of SAP R/3 which produced the IDoc etc.
Data records - A SD order can have any number of items)
Status records - It contains messages like 'IDoc created', 'The recipient exists', 'IDoc was successfully passed to the port', 'Could not book the invoice because.'

If you are creating an IDoc error report -

You might see the following two status messages -
IDoc status 51 - "Application document not posted"
IDoc status 52 - " Application document not fully posted"
IDoc status 63 - "Error passing IDoc to application"

Here is a list of transactions, you can use to understand -

• WE02 - IDoc List report

This report can be used to display and edit IDocs.

• WE19 - The IDoc document should not get proceesed.
• WE20 - Partner Profile

This transaction determines a processing code based on the partner profile identified by the control record of the IDoc.

• WE21 - Ports in IDoc processing

This transaction identifies an external port (RFC, File, ABAP-PI, etc.) that controls the IDoc flow to an external system.

• WE30 - IDoc Type Development
• WE31 - Development IDoc Segment
• WE32 - Development IDoc View
• WE41 - Outbound process code

This transaction links an outbound processing code specified in a partner profile to a function module.

• WE42 - Inbound process code

This transaction links an inbound processing code specified in a partner profile to a function module.

• BD87 - Inbound processing