OFP provided the ability to use random numbers. Of course they are not really random, there is a computer algorithm that generates each number from the previous one - they just appear to be random.
Algorithms like this usually need to have a starting value. That is what randomSeed is. There are many ways to cretae the random seed - it could be based on any or all of: time of day and date when the mission was last saved, time since last key stroke etc. etc. or none of the above.
This is all something that the OFP engine does. I have never editied a map but I would be surprised if randomSeed causes you a problem.