Words like “this” “these”, “it”, “their” “they” refer to other words. The words they refer to are nouns that have occurred previously. You will use these words a lot – there are plenty of them even in this short post! But often for readers, it is hard to be sure which noun the word is referring to. It may seem obvious to you, but it may not be obvious to your readers. If readers get the wrong idea they may be confused, and may completely misinterpret your point. Even if they can work it out by the context, doing so slows down their reading, can make readers lose the thread of what you are saying, and makes it unpleasant to read your work – the last thing you want!
This short posting by Barb at the Biomedical Editor website gives some examples and how to fix them in your writing.