。
In between studying, I watched TV.
Literal
Study [gen-の] interval [in-に] I [topic-は] TV [obj-を] watched.
合間 ('interval, gap between') is a noun specifically for the time between bouts of an activity — the short stretches when you're taking a break from something. 勉強の合間に ('in the intervals of studying') locates the TV-watching in those breaks. More specific than 休憩 ('rest break').