Your course is made up of different modules. Each module is a self-contained, formally-structured unit of study, with a clear and explicit set of learning outcomes and assessment criteria.
Some modules will be compulsory - you must complete these modules; while some will be optional - ones you can choose. Different courses will have different requirements on how many compulsory modules you must take vs how many optional modules you can choose to make up the correct number of credits.
Each module has a credit value; typically undergraduate students study modules with a combined total of 120 credits per academic year and postgraduate students study modules up to a total of 180 credits.