API
The agreed way one piece of software asks another for data or an action. It is the contract between the screen and the server.
What it tells you about a developer
API design skill means other developers depend on this person's decisions, which is influence beyond individual output. A developer whose public API or library other projects consume has passed the hardest review there is: strangers chose to build on their work. An API that survives versioning and growth without breaking the people building on it is a distinctly senior skill.
An API defines what requests a service accepts and what answers it sends back. Two programs, teams, or companies can then connect without seeing each other's code. Each request address is called an endpoint. REST is the most common style. GraphQL is a newer one that lets the caller name exactly the data it wants.