Many new laptops, such as the Macbook Pro, have only USB C ports. If you’re trying to connect any non-USB C device like USB 3.1, HDMI or Thunderbolt, you’ll need an adapter.

Legacy Adapters

Connecting an older device to a host with a USB C receptacle requires a cable or adapter with a USB A or USB B plug or receptacle at one end and a USB C plug at the other. Legacy adapters (i.e. adapters with a USB A or USB B plug) with a USB C receptacle are "not defined or allowed" by the specification since they can create "many invalid and potentially unsafe" cable combinations.

Adafruit sells a USB plug to USB A socket for legacy devices. These adapters don't magically give you USB C speeds into a USB 2.x device, and they won't give you the power negotiation capabilities because those pins aren't available on type A connectors. So it's strictly for devices that use 5V and up to perhaps 2.5A.

And for something bigger:

Audio Adapter Accessory Mode

A device with a USB C port may support analog headsets through an audio adapter with a 3.5 mm jack, providing four standard analog audio connections (Left, Right, Microphone, and Ground). The audio adapter may optionally include a USB C charge-through port to allow 500 mA device charging. 

A great number of these are marketed for cell/mobile phones. If you need one for a single board computer, be careful of the labeling. Most single board computer adapters are USB A. 

Also beware such adapters will not add audio capability to a microcontroller. Look for add-ons that are likely not USB specifically tailored to the microcontroller you are working with as there are several audio interface methods (DAC, I2S) to consider.

See Wikipedia for additional information.

Alternate Modes

Alternate Mode hosts and sinks can be connected with either regular full-featured USB C cables, or converter cables/adapters:

USB 3.1 Type C to Type C full-featured cable

• DisplayPort, MHL, HDMI and Thunderbolt Alternate Mode USB C ports can be interconnected with standard passive full-featured USB Type C cables. These cables are only marked with standard "trident" SuperSpeed USB logo (for Gen 1 cables) or the SuperSpeed+ USB 10 Gbit/s logo (for Gen 2 cables) on both ends. Cable length should be 2.0 m or less for Gen 1 and 1.0 m or less for Gen 2

Thunderbolt Type C to Type C active cable

• Thunderbolt 3 Alternate Mode with cables longer than 0.5 m requires active USB C cables that are certified and electronically marked for high-speed Thunderbolt 3 transmission, similarly to high-power 5 A cables. These cables are marked with a Thunderbolt logo on both ends. They do not support USB 3 backwards compatibility, only USB 2 or Thunderbolt. Cables can be marked for both Thunderbolt and 5 A power delivery at the same time.

USB 3.1 Type C adapter cable (plug) or adapter (socket)

• These cables/adapters contain a valid DisplayPort, HDMI, or MHL plug/socket marked with the logo of the required Alternate Mode, and a USB-C plug with a "trident" SuperSpeed 10 Gbit/s logo on the other end. Cable length should be up to 0.15 meters.
• Active cables or adapters contain powered ICs to amplify/equalize the signal for extended length cables, or to perform active protocol conversion. The adapters for video Alt Modes may allow conversion from native video stream to other video interface standards (e.g., DisplayPort, HDMI, VGA or DVI).