Computers use pseudo-random number generators (RNG) to create sort-of random numbers. These RNGs create the exact same series of numbers every time because computers are terrible at true randomness. Fortunately it's a series of numbers of infinite length and you can start anywhere in the sequence you want. What a seed does is tell you where in the sequence to start.
Of course, there is something of a chicken and egg problem here. How do you get a random seed if computers don't do random? One common way to do it is to use time in some way as a seed into an RNG that exists just to get you unique seeds!
But what does this have to do with creating a map? In Stonehearth, they use a function called simplex noise (which is more useful form of perlin noise) to generate the details. This function can create an n-dimensional shape (typically 3-dimensional for maps) that is infinite in size. It uses an RNG to do so. To ensure that each map is different, you'll want a unique seed for the RNG.
Simplex noise is a fairly complicated subject and depending on the input parameters (seed, octaves, amplitude, etc) can generate a vast variety of results. Also, you don't have to just use one set of noise. You can use many sets of noise layered on top of each other with differing parameters and functions to create some really elaborate psuedo-random results.