Lagrange Interpolation how does it work?

Question

I am making a program to find a polynomial given a set of data. I understand the summation of the formula. Given the image below can someone explain what the square looking symbol is next to the Li(x)? Can you explain the Li(x) function to me as well?

Lagrrange Formula

Answer 1

Perhaps you can understand it via this example on 3 points. This Desmos graph is interactive and you can create any interpolation on 3 points with it.