A rubric is a scoring guide that spells out what good work looks like before anyone is graded. It has three parts: the criteria you assess (argument, use of sources, structure), the performance levels (excellent, good, developing, beginning), and a short descriptor in every cell that says what each level of each criterion actually looks like.
A good rubric is short and specific. Three to five criteria that match the learning goal of the task, levels a student can tell apart, and descriptors that name something observable rather than a feeling. "Uses three relevant sources and cites them correctly" can be checked. "Excellent research" cannot. When a rubric is written like that, students can use it before they start, and your grading stays consistent from the first paper to the last.