To be awarded a honours degree, typically you need 360 credits, they are typically made up as follows:
120 credits QCF level 4
120 credits QCF level 5
120 credits QCF level 6
Some universities refer to the levels as 1, 2 and 3, Scotland use a different scale, but the principle is the same. To be awarded the credits, you need to achieve the minimum pass mark (typically 40% for undergraduate), meet any submission and attendance requirements. The credit and level of each module, represent the difficulty and content of the module.
Your degree grade depends on how well you pass each credit. Some universities may disregard the grades achieved at level 4 and 5, others may include 10% or more. It will be stated in your handbook. Where they don't contribute to your final degree classification, a pass must be achieved to progress.
Universities typically structure courses in multiples of 5 credits. So you may see modules allocated 10, 15, 20, 25, 30 and 60. If you have choice in modules, you need to make sure that that you meet the credit and level requirements of your course.
If you want to be sure of achieving a 2:1 or better, you need to ensure that your average mark at any level does not fall below 60%. Some universities, may have further requirements, that X percent of the modules must be above the threshold. Plus it looks better on your transcript if you don't have any anomaly lower marks.