Even if you haven't heard of data described as a "time-series," you've probably seen examples out in the wild. As the name suggests, a time-series is a representation of an event over a period of time. That could mean representing many different changes: the highs and lows of your curling ice temperature over a year, the number of cars that drive across a bridge every day — or, more relevantly, your application usage data, such as error rates over time or the growing number of activations per day. With enough data collected — that is, over a long enough time period — you can start to forecast future trends using time-series analysis.
There are many different ways to project trends about your application's behavior. Some systems are configured to perform a handful of tasks, while others are much more flexible, at the cost of a longer initial setup and administrative time. Your choice of tool depends on the goals you're trying to achieve.