For it to be in sync or to perform actions involving your phone, you need to be in Bluetooth range which is in the neighborhood of 33 feet/ 10m – more or less. The watch will fall back to wifi mode if out of this range when wifi is available. It could then perform more limited phone-related functions. When using your Apple Watch independently of your iPhone, keep this in mind if the connection drops out. A small, red phone icon with a line through it will appear on your watch face to tell you this.