A time series gives a relationship between two variables. Out of these two variables, one is time. More than often, Time series analysis is used to deal with data such as stock, weather analysis, sales, etc.
This is to build a visual template of the non-stationary data with respect to time at continuous intervals.
Time series models are the simplest longitudinal models, and longitudinal modeling is important in many fields, from modeling business processes to understanding the evolution of disease or social processes to predicting the weather.