Your @username is how people tag you in posts. It has 3 to 20 characters: letters, numbers and underscore. It is unique and case does not matter, so Trader and trader are the same name. Some names, such as admin, support and memewarzone, are reserved.
You can change it once every 30 days. Setting or changing it asks for a wallet signature, with no fee. Without a username nobody can tag you. Your profile also opens at /profile/yourname.