Some modules have one or more pre-requisite module(s), this is to ensure that you have the right experience and knowlege to complete the module. For example, to take PHY216 Galaxies in level 2, you must previously have taken PH104 Introduction to Astrophysics in level 1. Before choosing a module, look for any pre-requisites and only choose the module if you have taken these.
You can find lists of modules with pre-requisites and links to module information by following the links below.
Theoretical Physics (Maths modules)
If you are choosing your level 2 modules, or for level 3 of your MPhys, you might also like to consider which modules you might wish to choose in the subsequent year, to make sure you don't miss out any necessary pre-requisites. These flow charts of Astrophysics modules (modules from the Physics with Astrophysics degree which are available to Physics students) and Maths modules (Theoretical Physics students only) will help you.
Astrophysics modules available to Physics students
Maths modules (for Theoretical Physics students only)
Most Physics options have only Physics Core modules (which of course you will be taking) as pre-requisities. See this list of the Physics option modules.