This item is for calculating a numerical derivative, dy/dx. The "Differentiate (dy/dx) window opens by clicking this button. A polynomial equation is fitted to the straight data by the least-squares calculation. Then dy/dx is calculated by using the equation and the average of x. This is similar to so-called "moving-average".
"Order of polynomial"
A polynomial fit to a series of data is used to calculte this numerical derivative. This option is to select the order of the polynomial equation. Usually the fist order, in other words, the linear fit gives good results.
"Use every n points"
You can enter the number of data used to calculate one data point of dy/dx. When you select "10", the least-squares calculation is carried out to fit a polynomial equation to straight 10 points. Then the slope=dy/dx is calculated using the equation. The x value to calculate dy/dx is the average of the 10 points. The next dy/dx is calculated by shifting 1 point in the series of data. The number of total data points decreases by n-1 after this operation, where n is oder of the polynomial. Note that the number of straight data points should be greater than n in principle.