The SMA (Simple Moving Average) indicator displays the average of a certain number of closing prices in the price history. The number of closing prices included in the equation that creates the line is called the period (see article below to learn how to modify the period).
For example, if the period is set at 10 then 10 closing prices will be added together and then divided by 10 (producing an average). This equation is repeated throughout the price history and is represented by a line. A higher period will produce a more general line because it is using more data in the creation of the average.
To learn how to change this indicator's settings refer to the following articles: