When you join a Wi-Fi network, if the network's name has a lock icon next to it when you join it, then your data is encrypted between your device and the wireless access point. That means that other people on the network can't read your data. However, the administrators of the access point may still be able to read your data, if it isn't encrypted another way. Just because you had to enter a password to join the network doesn't mean your Wi-Fi traffic is private.