API: the agreement between two systems
Think of an interface that defines how information is exchanged, rather than copying a screen.
A shared language
An API lets one application expose particular information and operations through a controlled interface. An e-commerce application can use it to ask an inventory system for a product’s available quantity.
Rules matter as much as the connection
Required fields, date formats, permissions and error responses are part of the agreement. A successful connection alone does not prove that the data means the same thing in both systems; product codes must match too.
A first check
Trace one record: who creates it, how the next system receives it and which rule it follows. This helps you choose an initial integration instead of connecting everything at once.