This is the internal storage which contains the Android system, preinstalled apps, email, text messages, and cached data from the apps. You can also install apps that you've downloaded and capture photos, videos, and screenshots to this storage.
Use your storage card as a removable storage to store and access your media and other files on HTC One M9 or any Android phone, or set it up as an extension of the internal storage.
See Should I use the storage card as removable or internal storage?.
You can plug in an external USB storage device such as a USB flash drive or an SD card reader.
You need a special cable to connect
HTC One M9 and a USB storage device.