Some phones tend to automatically connect in "charge only" mode. To ensure the fastest possible communication and transfer speed, please change the connection mode to MTP (Media Device).
Swipe down on your phone, find the notification about "USB options" and tap on it.
A page from settings will appear asking you to select the desired connection mode. Please select MTP (Media Transfer Protocol). MTP basically lets you browse files and folders stored on your device, however, some phones may be required to be unlocked in order to enable MTP.
Wait for your phone to automatically reconnect. Some phone models (i.e. Huawei) have this option done a bit differently in the UI. After swiping down you´ll find the following notification, simply select "Files" and you´re done.
Android 6.0 and higher
Upon connecting, your phone will ask for permission for the PC connection to access data and files. Simply click on the "Allow" button and you´re all set.