An API (Application Programming Interface) is a technical “entrance” an application offers to other applications — for example “give me today’s orders” or “create a new customer”. The most common type is a REST API, where data is usually exchanged as JSON.
APIs let systems exchange information without people copying data by hand. Before planning an integration, it matters whether your applications have an API — or another proper way to get the data out.
Example
Our document management system iComp has a REST API, so an ERP or another application can exchange data with it directly.