Minecraft seeds are like codes, randomly generated numbers and letters, that if used will create a specific world terrain. Seeds can be shared with others as they form exactly the same landscape. Therefore seeds have become quite popular as if you end up in a world with very cool terrain, unique to any other, this you may want to share with others. Therefore there are websites only dedicated for this that lets you share your seeds with others.
To use a Minecraft seed you first need to Create A New World. Before actually creating it you must select Options and fill in the seed you want to create. In that field you can put in any number and letter you want and it will create a terrain based on the input.
Sometimes you can find seeds that really suits their names. An example of this is “Glacier” which back in the days spawned you in a winter landscapes with huge mountains all around you. Some people meant the Mojang team did this themselves, created seeds that suited their names, but it’s highly unlikely as nothing like that has been found in the source code as people have looked it through. That would be known as easter eggs and so far there doesn’t seem to be any in Minecraft. But we can of course not rule that one out as it’s just a theory based on what little we know.
To get an idea of why people want to use others seeds please look at the image below. As you can see the terrain is very different in each seed. Sure, you could maybe find all of these landscapes in one seed but the whole idea with seeds is that you spawn close to where these images are taken and don’t have to walk (or fly if in creative mode) long distances to get there.