Explain the purpose of your work, how you achieved that purpose, why you chose the style/form/features that you did and how they further your work's concept, who your intended audience is, how your topic links to the Advanced or EE1 course, what model/theoretical research was particularly influential in your process etc.
The idea is that you're reflecting on the finished product, so don't spend too much time saying "I chose this particular question but it didn't work so eight weeks later I stopped procrastinating and came up with a new topic ..."
It's a good idea to read some examples to get an idea of what a Band 6 reflection statement would look like. Ask if your teacher has any from last year or alternatively there's some up on here.