The Wi-Fi settings menu on Android opens a list of nearby Wi-Fi access points, along with Wi-Fi access points you have set up or used before.
While Wi-Fi is turned on, your device will search for nearby Wi-Fi access points, and if it sees one that it "remembers" - that is, you have connected to it before and it is still in the list of remembered access points - it will automatically connect. Your device will remain connected to that Wi-Fi access point for as long as it is in range.
While connected to Wi-Fi, your device will show a Wi-Fi icon in the notification bar, and all Internet data will be transferred over Wi-Fi instead of over a mobile network.
Accessing additional Wi-Fi settings
Additional Wi-Fi settings can be accessed from the menu button on your device. Some additional settings specific to certain access points can be accessed by long-pressing on one of the visible access points.