The y bar symbol is used in statistics to represent the sample mean of a distribution.
The Latin letter y is used to represent a variable or coefficient. Most commonly, y is the vertical dimension in the 2D cartesian coordinate system.
The combining macron is a unicode character used to draw a macron (horizontal bar) over the symbol it is combined with.
The sample mean, also called the arithmetic mean, is the average of a sample space. To calculate the sample mean, sum all the data points in a sample space and then divide by the number of elements.