You are then asked to choose one of two options. Pick one that would best suit your needs.
Keep my Files: This will keep any files you have stored in the windows Documents, Pictures and Videos folders etc. This will result in a fresh install of windows but keep all your files.
Remove everything: This will completely refresh the windows OS, free from any files or apps that where on before, any files stored on other drives or partitions will remain. Make sure you back up any data you want to keep before choosing this option.