In a wireless (or wired) connection or network, latency refers to the delay between a signal or information originating at the transmitting end, and when it is received at the other end.
Latency is a combination of time required to send across a distance (usually close to the speed of light) and the processing time required at each end.
"High" latency refers to a longer delay; "low" latency refers to a shorter delay.
High latency can be caused by several factors. One is long distances (such as satellite or transcontinental links). Different types of digital connections add various amounts of latency with encoding, compression, and processing at each end, and sometimes also at points in-between. Additional latency can be added if a connection spans multiple types of networks.
Latency is one factor that can affect their perceived "speed" of a data connection, although it is distinct from bandwidth / throughput.