Understand How a Wireless Network Works

Going wireless generally requires connecting an internet "access point" – like a cable or DSL modem – to a wireless router, which sends a signal through the air, sometimes as far as several hundred feet. Any device within range can pull the signal from the air and access the internet.

Unless you take certain precautions, anyone nearby can use your network. That means your neighbors – or any hacker nearby – could "piggyback" on your network or access information on your device. If an unauthorized person uses your network to commit crime or send spam, the activity could be traced back to your account.

source: https://www.consumer.ftc.gov/articles/0013-securing-your-wireless-network