A transitional year can happen at two times - during junior high or secondary school, kids can take a year off studies in order to do some kind of work experience thing. For example, they can observe in hospitals, sort of talk to OTs or shadow someone for a couple weeks - and they do "rotations" of these in a field they might find interesting. They also have certain requirements they need to fulfill.
Or, a lot of high school/secondary school graduates take a year off between high school and college to go travel or figure out what they want to do/work a bit. This is more often called a "gap year." (Prince William did one and travelled).
It's a lot more common in europe to take advantage of these times, versus going straight through the progression of academic studies like we do in the US and Canada without a break.