The most essential question would be, what do we mean by time series data? Of course, we have heard about it on several occasions. Perhaps we can define it? Sure we can. Essentially, a time series is a collection of observations made sequentially in time. Note that there are two important key phrases here—a collection of observations and sequentially in time. Since it is a series, it has to be a collection of observations, and since it deals with time, it has to deal with it in a sequential fashion. 

Let's take an example of time series data:

The preceding screenshot illustrates solar energy production (measured in Gigawatt Hours (GWh)) for the first six months of 2016. It also shows the consumption of electricity on both a daily and weekly basis.