The idea behind Ubiquitous Computing is that the third wave of 
computers is coming (or already here).

The first wave was Mainframe Computing: computers are large machines 
only found in small quantities at large companies, and have several 
users each. The second wave is Personal Computing: every home or at 
most person has a "desktop" computer. The third wave is Ubiquitous 
Computing: there are (many) more computers than users. Mobile Computing 
that we are seeing today, where users have several devices (computer, 
tablet, phone, car system...)  is an intermediate between the second 
and third wave.

"Real" Ubiquitous Computing can encompass a lot of things: sensor 
networks, intelligent dust, robotics, augmented humans... Basically it 
is the vision of a future where computation is everywhere.

Regarding programming, the shift is that, in earlier waves, an 
application was running on a computer. In Ubiquitous Computing this is 
no longer possible, as there may well be fewer applications than 
devices. An application runs in *several* computers. This is why the 
idea of the Cloud is (kind of) related to Ubiquitous Computing.

So learning how to program for Ubiquitous Computing is first and 
foremost learning how to write distributed algorithms. I have no idea 
what your current level is, but if you are at least an intermediate 
level programmer a good start is the problem of consensus and the Raft 
algorithm. You would also benefit from learning Erlang and/or playing 
with systems like 0MQ / nanomsg.

Other things will be important though. You cannot have hundreds of 
devices per user that each consume 50W of power, so some of the 
programming will have to be energy-conscious. That means it will be 
very low-level. Odds are that it will be closer to circuit design 
actually, so I'd say learning about things like FPGAs and VHDL could be 
a good thing. Playing with Arduinos as well.