geu poldeo-e deureogamyeon eoje daunrodeuhan jaryoga isseo.

그 means that.

In Korean, the basic demonstratives are:

• 이 = this (near the speaker)
• 그 = that (near the listener, or already known from context)
• 저 = that over there (far from both speaker and listener)

So 그 폴더 means that folder. In real conversation, 그 is also very commonly used for something already mentioned or understood, even if no physical pointing is involved.

Here 에 marks the destination or target location of 들어가다 (to go in / enter).

• 폴더에 들어가다 = to go into a folder
• 방에 들어가다 = to go into a room

With 들어가다, Korean usually uses 에 because the focus is on entering a place.

By contrast, 에서 is more often used for the place where an action happens:

• 방에서 공부하다 = study in the room
• 폴더에서 찾다 = find it in the folder

So in this sentence, 그 폴더에 들어가면 means if you go into that folder or when you open/access that folder.