1. What does it mean?
공강 is time when you do not have a scheduled lecture. University students use it for breaks between classes and for days left class-free by their timetable.
Also written or said as gonggang
2. The nuance
A scheduled class being canceled is more specifically 휴강. 공강 also does not necessarily mean free of homework, meetings, or other obligations.
3. Examples
“두 수업 사이에 공강이 길어서 도서관에 있었어.”
I had a long gap between classes, so I stayed in the library.
“내일은 공강이라 과제를 몰아서 하려고 해.”
I have no classes tomorrow, so I plan to catch up on assignments.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.