There have already been studies that prove labelling icons enhances usability on desktop applications. This is why Microsoft Outlook started labelling its icons.
Does the same logic apply to mobile apps? There is limited screen space on a mobile phone. Adding labels to every icon would harm the aesthetic design. I'm thinking of designing my application without many labels, like this popular app:
My theory is that even though users don't necessarily know what all the icons do (three lines, two people, smiley face, plus sign), they will try tapping them to learn what they do. My feeling is that mobile applications are more playful and inviting than desktop applications, so users are not afraid to explore unknown icons. The desktop is a cold hard machine for serious business, whereas your phone is always with you and you interact with it every hour.