# Monte Carlo: what is a seed?

I have reached a chapter in the notes I am following where a program written in C++ is using a Monte Carlo method to estimate $\pi$. It mentions a 'seed', but does not say what this is. I have tried finding out what a seed is online, but I only seem to get results about 'why you should/shouldn't use a fixed seed', and unfortunately they have left me none the wiser as to what a seed actually is.

What is seed in this context?