Go: What is context?

1ctx := context.Background()
2ctx, cancel := context.WithTimeout(parent, 5*time.Second)
3defer cancel()

1func handler(w http.ResponseWriter, r *http.Request) {
2    ctx := r.Context()
3    dbCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
4    defer cancel()
5    
6    if err := queryDB(dbCtx); err != nil {
7        if errors.Is(err, context.DeadlineExceeded) {
8            http.Error(w, "timeout", 504)
9        }
10        return
11    }
12}