The delay between sending a request and receiving a response. Like the time it takes between asking a question and getting an answer.
Online gaming requires low latency - players notice if there's more than 50 milliseconds delay between their actions and the game response.
Latency is a performance metric, not a standalone cloud service. All major cloud providers offer tools to measure and reduce latency (for example via monitoring, CDNs, edge locations, and private networking), but there is no single one-to-one “Latency” service name.