An interface is a shared boundary between two components. Most systems require connections with other applications, hardware and peripheral devices to function properly. Interface Analysis is a business analysis elicitation technique that helps to identify interfaces between solutions/applications to determine the requirements for ensuring that the components interact with one another effectively.
According to BABOK, interface types range from user interfaces (human beings interacting directly with the system); interfaces to and from external applications; and interfaces to and from external hardware/gadgets.
To put it simply, requirements that define how human beings interact with the system; how applications link to other applications and how hardware links to applications need to be defined for effective functioning of the system. Interface analysis helps in discovering the requirements needed to integrate software into its new environment.
Interface analysis can also help in determining requirements for interoperability and exposing interfacing stakeholders early on in the project. The last thing you want is to discover at the eleventh hour that there is an application from which the new system will require data.
A quick way to gain insight into interface requirements is by reviewing the existing system and building a context diagram which displays at a glance, the entities that send data to and receive data from it at a high level. The context diagram can then be broken down into data flow diagrams for more detailed analysis.