If your feedback says: "Ref?" or "Reference?" or "Source?"
Your tutor wants to know where the information in your writing is coming from - the reference you have used, the source of the information
Answer
Context
Much of your assessment will require you to read academic books, journal articles, and other academic sources of information, and reference (cite) them in your writing. Your tutors will want to know what you have been reading.
Referencing is a way for you to show what you have been reading, where your ideas are coming from, and which evidence you are using to back up your arguments.
Terminology
If you say something in your writing and don't include a reference, your tutor may want to know what you have been reading, what the evidence is to back up your claim. They will sometimes write "Ref?" or "Reference?" or "Source?", or more detailed feedback such as "Where is this idea coming from?" or "You need to provide evidence for this" or "It is unclear who said this".
Sometimes a tutor will use the term "Ref?" or "Reference?" or "Source?" if you have included a citation in your writing, but you haven't included it in your reference list. Sometimes they will write "Bib?" or "Bibliography?" or "Reference list?" to indicate a missing reference from your reference list.
Whenever your tutor sees a reference within your writing (called a citation)
If you don't include a citation in your writing, and it is unclear where an idea in your writing has come from, a tutor may assume that what you are writing is your own thinking. This is typical when you are providing your own analysis or interpretation, or when you are evaluating something. It is therefore okay (and expected) that your own response to what you have read do not have any further references, as long as it is clear what your analysis, interpretation, arguments or evaluation (that is, your own thinking) is based on in the first place.
Meaning: it must be clear what you have been reading to get to your own thinking. This is what is meant by 'evidence-based writing' and is different to simply stating your own opinion that isn't based on any evidence.
When assignments are submitted via Turnitin, it will be clear when sentences or whole sections have been copied from somewhere else (including other assignments, your own or other people's). If there is no reference, this may be taken as an attempt to pass another person's thinking off as your own. We call this plagiarism, and it is a serious academic offence. Accidental plagiarism, where there is no attempt to cheat, but where a reference is missing, is called Bad Academic Practice. It is important, therefore, to understand when and where references are expected and required. It is your responsibility as a student to learn this.
If you want to learn more about good academic practices:
There is a detailed "Guide to Good Academic Practices".
The referencing guide for DMU students covers different referencing conventions. Depending on which course you are studying, you may be following the Harvard, APA, Chicago, IEE, MHRA, OSCOLA or Vancouver referencing conventions. Each of these has its own tab in the guide. Harvard is the most commonly used referencing system, but you will need to check your subject guide to make sure you are using the right one.