Reading comprehension is a culmination of basic literacy skills, converging into the understanding of the text written upon a page.  Students must know and understand our language of English to comprehend text (language comprehension).  They must also be able to decode and be fluent in word reading and also have a robust knowledge of our world, as well has enhanced vocabulary.  There are other factors such as attention and working memory that can also play a role in the ability to obtain comprehension, but it is not just a separate skill that can be targeted for remediation.