What problem does context.Context solve, and what are its four methods?

context.Context carries three things across API boundaries and goroutines for the lifetime of one request or operation: a cancellation signal, an optional deadline, and request-scoped values. Before it existed, each library made up its own "quit channel" and timeout handling, so cancellation could not flow cleanly from an HTTP handler down to a DB driver.

type Context interface {
	Deadline() (deadline time.Time, ok bool) // ok == false: no deadline
	Done() <-chan struct{}                  // closed on cancel/timeout; nil if never cancelable
	Err() error                              // nil until Done closes, then Canceled or DeadlineExceeded
	Value(key any) any                       // request-scoped lookup
}

Key properties: contexts are immutable. You never change one; you derive a child with WithCancel, WithTimeout, WithValue and so on, which builds a tree. Cancellation flows down the tree, never up. All methods are safe to call from many goroutines at once.

What the interviewer is looking for: that context is for request lifetime and control flow, not a general dependency-injection bag. Also that Done() returning nil (for Background()) is fine inside a select, because receiving from a nil channel blocks forever.