About USB on Mac
Your computer has USB ports 
that let you easily connect external devices, such as a keyboard, mouse, trackpad, printer, iOS devices, or a digital camera, without needing to restart your computer. The USB ports provide some power, so most devices don’t need their own power cords. You may be able to purchase adapters that let you connect non-USB devices to a USB port.
Note: Your Mac may also have one or more USB-C ports. See About USB-C on Mac.
About USB devices and portable computers
If you have a portable computer, USB devices can cause your computer’s battery to become depleted faster. If you are using the device for an extended period, it’s a good idea to connect the computer’s power adapter.
About disconnecting USB storage devices
Before you unplug a USB storage device while your computer is on, eject the device by dragging it to the Trash.
About connecting multiple devices
You can connect more than one device to a USB port by using a USB hub. Connect the devices to the USB ports on the hub, then connect the hub to the computer. Most USB hubs have a power adapter and should be plugged into an outlet.
Be sure to connect USB 3.0 SuperSpeed devices to a USB 3.0 SuperSpeed HUB, USB 2.0 Hi-Speed devices to a USB 2.0 Hi-Speed hub, and so on.