If you type in anything else (like letters), it will be converted to a number. You should also know that a seed is always a number (up to around 20 digits). When loading a seed from a level, the seed will automatically be stored as a level.
Levels allow you to store and reuse seeds on this website, without having to load your savegame every time. On Windows you can use %appdata%\.minecraft\saves to get to that folder. You can find the savegames in the saves folder of your Minecraft installation. Level.dat is a small file located in the folder of every Minecraft savegame. The latter can be done by clicking on 'Load from Save.' and selecting your level.dat, or by drag&dropping the level.dat file into your browser window. You can either type it in manually, or you can load it from your savegame. The first thing you should do is select a seed and version.
