Each module is constructed with details on a particular subject, followed by a summary. Work your way through all the modules, it doesn't matter whether you understand everything the first time. It's important to appreciate that working through the course isn't a memorising exercise, where everything is learnt word for word. It's about building an understanding through small steps, by learning to walk before trying to run, as the saying goes.
Once you've been through the course, repeat the process, starting from module 1 again and work all the way through to the last module, till you feel confident enough to have understood a large part of the information.
It's perfectly acceptable to keep going back over a module if it's not understood, as long as you're comfortable in how you're using the course, the learning and understanding should come naturally.
Try to use the 80/20 rule, whereby work towards understanding eighty percent of what's in the modules. The other twenty percent will come eventually, so there's no need to stress about learning it all, in one go.
Sometimes something may not be easily understood in a particular section. Instead of re-reading it and letting it mull over and over in your head, forget about it and do something else, like watch TV. Then come back to the section that's been troubling you the next day.
This can help in digesting the information easily, as trying to overload when concentration levels are low, only leads to frustration and poor information retention.