Latency is the delay between when something starts and when you experience its effect. When you click a link on a website and have to wait a second before the page loads, that waiting time is latency. When you're playing an online video game and your character seems to move a split second after you press the button, that's latency too.

During the latency period, something is happening (your computer is sending signals, the server is processing your request), but you can't see the results yet. Think of it like the moment between flipping a light switch and the bulb actually lighting up. In a well-working system, that delay is so tiny you barely notice it. But when latency gets longer, it becomes frustrating.

Latency matters in many situations. In telecommunications, high latency makes phone conversations awkward because there's a noticeable gap between when someone speaks and when you hear them. In medicine, doctors might talk about the latency period of a disease (the time between when you're infected and when symptoms appear). In music, latency between pressing a piano key and hearing the sound would make playing nearly impossible.

Low latency is usually desirable because it means things happen quickly and responsively. High latency means annoying delays that slow everything down.