You have probably seen what I am talking about.  Take your remote control or example, you have been changing through the channels at some point in time only to have someone walk in front of you.  If you were holding the button down with the channels changing automatically, they will suddenly stop.  However, if you were pressing the button each time you wanted to go to the next channel, while someone was in front of you, then you would be unable to pass the beam to the receiving device.  The reason this happens is because remote controls and other similar infrared devices use beams of low level light that cannot be picked up by the human eye.  Because there is something blocking the patch between the remote and the receiving device, this beam of light can't travel to it's destination.

However, with the advent of bluetooth technology, you no longer have to worry about having objects or people within the line of sight for your devices.  Bluetooth does away with this by using wireless signals that bounce around the room in an attempt to grab a connection.  Once the connection has been made, the signals become super focused and the bluetooth device will release itself from discovery mode.  You now are able to walk around the house, having walls, people, furniture, even the dog standing in the way without having to worry about losing the signal on your call.  Whereas before these types of objects would cut the signal off, having a bluetooth capable receiving device within range, usually around 5 to 10 feet, you are going to get the signal regardless of what is around.

There are still some exceptions to this, with certain metals causing interference with bluetooth signals.  Microwave ovens typically cause bluetooth signals to drop, so if you are going to be around a microwave while you are on the phone make sure that the person knows you may fade out for a second, or will be unavailable.  As long as you have the actual receiver located away from the metals or the microwave oven you will be alright.  Getting in range of the microwave with the bluetooth headset on will cause interference in the signal, but it will resume again once the microwave shuts off, or you move out of reach of the waves being emitted by it.