이것 - this
example sentence: 이것 뭐야? - what is this?
그것 - that
example sentence: 그것 뭐야? - what is that?
저것 - that (over there)
example sentence: 저것 뭐야? - what is that (over there)?
이것 (this) is right there next to you or what your holding, 그것 (that) is what is next to the person you're talking to and 저것 (that (over there) is away from both of you that's why it's like saying that over there
When you aren't adding something behind 이 you add 것.
이, 그, 저 are like prefixes. You can add them at the beginning of a word like 고양이 (cat) to say "this cat"
것 is kind of like saying thing. this thing, that thing, that thing (over there) in some cases.
hope I helped and didn't confuse *.*