And "个" is one of the most frequently-used measure words in Chinese. But actually not all the individual things can be measured using it. For example we use "条" for dogs,"头"for cow,"只"for ducks or birds,"台"for machines or technical equipment and so on. And I think usually things which you can grab with one hand can all be measured using "个". Measure words are a little difficult to learn honestly cause we've got too many.

那个 is that,这个 is this,个 is a

Thank you:)

It is a measure word when you use it alone. For example when you say "an apple" you have to add a "个" between "an" and "apple" so the translation would be "一个苹果". And in other situation you can just simply translate "那个" into "that" and "这个" into "this" when you use it in combination. :)