Some phones tend to automatically connect in "charge only" mode. To ensure the fastest possible communication speed and transfer speed, please change the connection mode to MTP (Media Device).
You can follow these steps in order to do it.
1. Swipe down on your phone and find the notification about "USB options". Tap on it.
2. 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 requires to be unlocked in order to enable MTP
3. Wait for your phone to automatically reconnect. Your notifications should then look like this.
Some phone models (i.e. Huawei) has this option done a bit differently in the UI. After swiping down you´ll find the following notification. On it simply select "Files" and you´re done.
For Android 6.0 and higher users:
Upon connecting, your phone will ask for a permission for the PC connection to access data and files. Simply click on the "Allow" button and you´re all set.