Generally, Physical Computing is the design and engineering of tangible interfaces with which to interact with various forms of computing. It has roots in Human Computer Interaction (HCI), but given the inter-disciplinary nature of the field, other simultaneous concerns such as interaction design, form (aesthetics), ergonomics are involved. Consequently the skills needed to work in Physical Computing include a converging practice with digital media, coding, electronic engineering, and fabrication.
Here's another definition of Physical Computing, as described in Wikipedia.
As the site name implies, this section will not discuss much, if any, of the design-centric or fabrication side of things, but instead focus on the electronic prototyping, and coding technologies. Most of my physical computing lessons centre around the Particle Photon, Arduino (and its variants), as well as the ESP8266 low-cost chip, itself also capable of being programmed via the Arduino IDE.
If you're looking for introductions and primers to these platforms, check out the Curated Links section.