What is the “invisible gun” mentioned in chapter 10 of Great Expectations?

“He was a secret-looking man whom I had never seen before. His head was all on one side, and one of his eyes was half shut up, as if he were taking aim at something with an invisible gun.”

This “invisible gun” is mentioned several other times and it’s really confusing me (and also I have to write something about this so I really need to know)