An undergraduate philosophy journal is a philosophy journal (an organization that publishes collections of papers) that only accepts submissions from undergraduates (people pursuing a bachelor’s or associate’s degree, or something like that). They are usually run by the students in a philosophy club at a university, sometimes with the assistance of professors, graduate students, staff members, or others. Ideally, they will put out a call for papers (CFP) each time they are getting an issue together, which often happens yearly. Details vary from journal to journal.
Here are some benefits of publishing in one of these journals:
The above list covers the typical benefits, I think. Notice that the list does not include “it makes you a more attractive candidate to graduate schools if you apply to them.” At least for philosophy graduate schools, typically your chances of admission are largely unaffected by whether you have published in an undergraduate journal. If there’s any impact, it’s very minor. This is because undergraduate journals typically have standards that are low enough that their acceptance of your paper doesn’t communicate useful information about your philosophical skills. If the paper is a good one, you’ll use it as your writing sample, and so the admissions committee will use its own judgment about its quality. Publishing in an undergraduate journal does demonstrate your interest in philosophy, but probably lots of other parts of your application will accomplish this, so the journal publication won’t matter very much.*
Of course, polishing your paper in order to submit it to the journal will make your paper better. And if you have a very good paper, that can serve as your writing sample for your application to graduate schools. But that has nothing to do with whether the paper gets published in an undergraduate journal. From the point of view of graduate school, publishing in an undergraduate journal is not helpful (or harmful).
This means that unless the things in the list above sound great to you, it’s probably not worth your time trying to publish in an undergraduate philosophy journal. But if those things do sound great, then go ahead! There are no downsides except from the time investment, and, if your paper is rejected, perhaps a blow to your ego. (There is one possible downside: if the paper is amazing, or it could be amazing after some more time spent working on it, and if you are thinking about pursuing graduate school, it might be better to save it so that you can publish it in a real journal. I thank Julia Staffel for noting this.)
Different journals will have slightly different procedures, but the process will be broadly the same for all of them:
* This is my view and lots of people agree with me. But, not everyone does! Graduate admission is not an exact science. Some people may be impressed that you published in an undergraduate journal in a way that helps your admission chances. I suspect these people are in the minority, especially when it comes to the people on admissions committees for philosophy graduate programs. But I am not 100% sure.