This IOTA cryptocurrency seed generator is fully client-side (JavaScript) software. It means, your data is processed in your web browser by your computer/phone (so it does not leave your system) - in the opposite to server-side applications, where your data is generated on a distant server (who know where, by who know who) and it's sent to you around the world (often in an unencrypted form).
Use Paranoia Text Encryption to encrypt and securely store your cryptocurrency seeds, keys, and mnemonics.
This tool is based on Stanford Javascript Crypto Library (SJCL) - more details: random.js