what does random seed value mean in weka?
Seed is just a value by which you can fix the Random Numbers that are being generated in your task. Also, this is a general concept and not just for weka.

So, here random numbers are being used to split the data. Now if you run the code without fixing any seed, you will get different splits on every run. But if you fix the seed to some specific value, you will get the same split every time. This is useful when you want to make your scores reproducable.