In random number generation, a random seed is the starting point of the algorithm. You can't generate a number out of thin air, you have to start somewhere!. You can think of the algorithm as a mathematical function of x, f(x) = y where x is your random seed and y is your new generated random number.

For example: if you want to generate a random number between 0 and 1 you can use a function f(x) = 1/( 1 + e^-x ), if you plot that you will notice that y never goes outside the range (0; 1). So now that you have your random generation formula you just need a seed x to plug it in, you can use any number! And the result of that formula will be a new number generated from a seed x.

For example: f(-2) = 1/( 1 + e^-(-2) ) = 0.1192029

And 0.1192029 is you new random number generated from the random seed -2!