Relative pronouns are pronouns that point back to nouns or ideas. This/These and That/Those are relative pronouns. For example,

At a party, a guest says "This is great!" This=party
Pointing to a pile of shoes, a boy says, "Those are mine." Those=shoes

What you'll notice is that these words only make sense if you can understand the context in which the words are being used. Which means, you, the listener, have to figure out what "this" and "those" refer to based on where you are, what you are doing, what the speaker is pointing to, etc. You need more information in order to make sense of those words.

In writing, we don't have all those other aspects of context to help us figure out what a writer is referring to. We aren't in the same room. The writer can't point through the pages. All we have for context is what is on the page, and sometimes that isn't exactly clear.

What does "that" refer to in the previous sentence?

Hopefully, you understand that it refers to "context," an idea mentioned in the first part of the sentence. But in order to be really clear in writing, we need to attach a clarifying word:

All we have for context is what is on the page, and sometimes that context isn't exactly clear.

By adding the clarifying word, I can make sure that my reader is not confused.

These clarifying words are one of the most useful tools we have for ensuring our use of relative pronouns make sense to the reader.

So, try to identify all the places where you use this/these and that/those, then make sure you add clarifying words so that the reader is sure to understand what you mean.