Reflection The effect of an RF signal meeting a dense, reflective material, such that it is sent in a different direction
If an RF signal traveling as a wave meets a dense reflective material, the signal can be reflected just like a light wave is reflected off of certain surfaces. In a wireless LAN scenario, the wave will reflect off metal surfaces such as filing cabinets etc or off windows. There would also be some level of reflection from walls and in outdoor deployment from the earth, buildings and water.
The reflection is a copy of the signal and if it is received along with the original signal (straight path) then it is likely to be out of phase. This causes the net received signal to be less. This is known as multipath. Prior to 802.11n, multipath was always detrimental. In 802.11n/ac, multipath has been harnessed to improve the performance of the transmission/reception train.