It's not simple thing. First of all the word "это" can be used instead of the verbs "is/are" which are absent in Russian language. Like: These are pens. = Это ручки. Apples are fruits. = Яблоки - это фрукты. Salmon is a fish. = Лосось - это рыба. But when you use it in meaning "this/these" (pointing pronoun) you must conform pronoun "это" with the object e.g.: These pens. = Эти ручки. These apples are fruits. = Эти яблоки - фрукты. This pen is blue. = Эта ручка синяя. This apple is green. = Это яблоко зелёное.

When you point at something in Russian language you always use 'ETO'. For example, chto eto? eto - sumka/sumki, eto - korobka/korobki. You use 'ETI' on plural words in Russian when you add some action or description to it. For example, ne trogai ETI ruchki (don't touch these pens).

"Это" - when you point at something to define "what is it?", no matter if it is one object or a set of objects. "Эти" - its when you identifiying it by question "which one(s)" - i.e. these ones Examples: 1) Imagine a man pointing with a finger to tomato and then to a set of apples. He will say: Это помидор, это яблоки - this is tomato, this is apples. 2) When he wants someone to give him these apples he will use: дайте мне эти яблоки - give me these apples.

"это ручки" - when to show "эти ручки" - when to specify

эти ручки = These pens (grips) это ручки = These are pens (grips) For example: These pens are red = Эти ручки красные. These are pens (not pencils) = Это ручки (не карандаши)