This is the 8602nd most frequent Korean word.
가방에
"가방에" means "in the bag" or "to the bag."
The phrase '가방에' functions to indicate the location where the book is being placed.
가방에 책을 넣었어요.
I put the book in the bag.
Here, '가방에' is used to specify the location of the pen.
가방에 볼펜이 있어요.
There is a pen in the bag.
'가방에' in this sentence implies entering into the bag as a location.