Most wireless routers don't turn security on by default. Enable encryption to prevent snoops from entering your network and perhaps capturing your traffic and passwords. For a home or small office, use at lease WPA2-Personal (PSK) security, in which you create a password on the router and then enter it into computers and devices when connecting. For a larger business, choose WPA-2-Enterprise security, which uses an authenication server or hosted service and requires users to have unique usernames asnd passwords.
This is by far the best way to secure your wi-fi network and I encourage you to use the same protection to ensure you're covered!