Latency is the time it takes for data to travel from your device to a server and back again. It's measured in milliseconds (ms). A latency of 10ms means a tenth of a second delay; 100ms feels noticeably sluggish; 200ms or higher causes visible lag in interactive activities.
Every network link introduces latency. Your device sends a request, it travels through your internet service provider's infrastructure, crosses cables or radio waves to reach a distant server, then the response makes the same journey back. Each hop adds a few milliseconds.
Latency matters most for real-time activities. In online gaming, high latency puts you at a disadvantage because your actions reach the server after your opponent's do. Video calls stutter and feel unnatural above 150ms. Web browsing and email tolerate higher latency because they don't demand split-second responses.
Real-world example: you're playing an online multiplayer game with 50ms latency versus a friend with 150ms latency. When you both press the shoot button at the same instant, your action registers 100ms sooner on the game server. That's enough to win the firefight.
When buying broadband, check the latency figures your provider quotes, not just download speed. Fibre-to-the-premises (FTTP) typically delivers 10, 30ms latency. Standard copper ADSL might reach 40, 80ms. Mobile networks vary wildly from 20ms to 100ms depending on signal strength and network load. For gaming, streaming, or video calls, aim for latency below 50ms if possible. For casual browsing, anything under 100ms is acceptable.