Sometimes students have a hard time reading a text because it contains so many unfamiliar words. Frequently teachers tell students to look up the words they don't know. But if there are a lot, it slows the reader down so much that the process of looking up all the words can discourage a student from reading at all.
Other teachers encourage students to keep reading when they run into a difficult word. Sometimes the meaning will emerge from the context. But if the student is an ESL student, for whom English is not the first language, this may not work. What is a good compromise between looking up every three or four words and reading very slowly on the one hand, and reading without any comprehension on the other?
Maybe a compromise would be to encourage the reader to read the text as well as she can without using a dictionary, underlining the words that she doesn't know. Then once she has a general sense of what the text is about, she could go back and start looking up some of the words. It may be clearer which words are the most important to understand, and which are not so important.
The truth is that fluent readers skip words and even whole passages when they read. I have found that the people who read the most give themselves permission to skim and skip. Other people have never gotten over their childhood training that one must read every single word of every text that one reads, and they feel guilty if they don't. But you are in charge of your own reading: read what you want to out of any given text! Even assigned texts can often be skimmed profitably. I think it's better, if you have limited time, to skim an entire text rather than just read a quarter of it or less.
Sometimes students come in the writing center having written an essay about a text that they never finished reading! I try to teach them to skim: to read the first paragraph in its entirety, the first sentence of every following paragraph, and the last paragraph in its entirety. Some students have never heard that this is "ok."