Definition from Wiktionary, the free dictionary
Confusingly linked from this.
이- as a prefix means this. It is part of a set as follows:
- 이 - this (near speaker)
- 그- that (near listener, or "what we were just talking about")
- 저- that over there (away from both of us!)
As a postposition particle, 이 goes with 가 as a subject marker. Ed Poor