An RF signal is transmitted in all directions from the transmitting antenna much like a water ripple travels in all directions from the point where a stone hits the surface of a pond. When that ripple hits a rock or the bank of the pond, the wave will bounce off and travel in a new direction across the pond.
The same thing happens with radio waves. They bounce off of trees, mountains, buildings and other objects. Since straight and reflected waves take different paths some arrive at the receiving antenna more quickly than others, causing cancellation due to differing phase relationships. This concept is called multipath. It results in lowered signal levels at the receiver and thus shorter useful distances for the link. Multipath is particularly a factor in interior environments where objects provide many different reflection paths.
There are many ways to combat multipath ranging from careful frequency and antenna selection, error correction, transmission redundancy, antenna diversity and the use of spread spectrum and frequency hopping.