API (application programming interface)
An API is an agreed way for one program to ask another for data or to tell it to perform an operation. Systems exchange information by themselves, in a format they have agreed on, instead of waiting for someone to export a file and upload it by hand.
A company sees an API through its effects, not its code. The shop sends an order to the ERP and gets a document number back, the WMS returns stock levels, the carrier system accepts a shipment and returns a waybill number, and an invoice goes to KSeF, Poland's national e-invoicing platform, straight from the program that issued it. Comarch ERP XL, enova365, SAP Business One, BaseLinker, InPost and most newer systems have a documented API.
The trouble starts where there is no API, or where it covers only part of the data, and in Polish companies that happens often: older ERPs, industry-specific programs bought years ago, versions the vendor no longer develops. What is left is work on exports: a CSV or XML file dropped onto an FTP server at a fixed hour, sometimes a read straight from the database, and writing back through the vendor's own import, because writing into somebody else's database usually costs you support.
Whatever the connection looks like, two things are easy to leave out of the estimate. A queue with retries, because the other side is sometimes down, overloaded or capped at so many calls per minute, and the document has to arrive later rather than vanish. And error monitoring that notifies a named person: an integration that breaks quietly only surfaces at month-end close.
You will recognise the problem if the same information is typed into two places, if someone pulls a report from one system every morning and pastes it into a spreadsheet for another, and if the question "which stock figure is the real one" comes up regularly.