Latency is the delay between a frame change on the source side until the same frame shows on the screen side, often referred to in computer terms as the total round trip time it takes for a data packet to travel. Latency is generally measured in milliseconds (ms) and is unavoidable due to the  infrastructure and the processing time on the source side and the processing time on the image side. Latency depends on several aspects of all the devices in your setup and can vary if any of them are changed or a device is added/removed.