Random numbers in Matlab are not random. They are pseudo-random: they are based on deterministic algorithms.
You can consider the seed as the starting point for the random number generation.
If you use the same starting point, you will get the same sequence of random numbers.
Just do an experiment.
Call rand two times. You get two different numbers.
Start Matlab. Call rand. Start Matlab. Call rand. You will get the same number: that's because the seed has been reset.