Lane Emden Equation is this ordinary differential equation.
The "$\xi$" is independence variable and means radius of a star. The "$\theta$" is a function depend on $\xi$.
The value of $\theta$ equals 1 in the center of the star, and equals 0 in the surface.
The "$n$" is a parameter that characterizes the equation and called polytropic index.
Lane Emden Equation is very important for astrophysics.
If you want to know details of this, please see Wikipedia!