What do the terms "CPU bound" and "I/O bound" mean?

It's pretty intuitive:

A program is CPU bound if it would go faster if the CPU were faster, i.e. it spends the majority of its time simply using the CPU (doing calculations). A program that computes new digits of π will typically be CPU-bound, it's just crunching numbers.

A program is I/O bound if it would go faster if the I/O subsystem was faster. Which exact I/O system is meant can vary; I typically associate it with the disk, but of course, networking or communication, in general, is common too. A program that looks through a huge file for some data might become I/O bound since the bottleneck is then the reading of the data from disk (actually, this example is perhaps kind of old-fashioned these days with hundreds of MB/s coming in from SSDs).