It allows you to send commands and get into your files. SSH can be useful to fix your if your interface goes missing or messes up (it's certainly saved me several times). But its entirely optional and can be uninstalled.
SSH based tools can be incredibly secure when setup properly, (after all, one of the S's stands for SECURE

). The default install has a preset password installed which you can easily (and should) change. Or you can setup private key pairs to login, there's plenty of tutorials on the web.
Can I ask why you are so worried about it? Some people have had their phones 'hacked' in the past because they were too stupid to change the default password.