Upload your SVG file to begin. A temporary (you have been warned!) PNG rendering of your SVG will be generated, at 1:1 size, and exactly how the Wikimedia servers would render it, warts and all.
Additionally, if you have ticked the 'Check this file for errors' box, then an output showing any automatically detected (possible) errors will be generated. This is designed for files known to be causing errors; it is therefore inclusive and will give more output than is normally necessary in order to ensure that the problem is found. Some errors are fixable using a GUI like Inkscape; others will require you to edit the file using a text editor like Wordpad. Since optimisation tends to strip out clutter, it can also help in finding the error (it may even fix the problem in some cases).
The 'Have a go at optimising this file and show me the results' optimises the text of the SVG using the general purpose "scour" library and an experimental custom one designed by me with MediaWiki in mind. The PNG rendering should (hopefully) remain unchanged. A display is rendered so you can check this yourself. Note that at the present time, optimisation does not significantly affect the size of the rendered PNG, and is therefore only useful as an added bonus, rather than as a cause for uploading a new version of a file.
Please be patient, the process can take some time, particularly optimisation of large files, which can take minutes. Some filenames represent a possible security risk and will be blocked, I'm sure you understand. Queries can be directed to me on-wiki or to my username at Gmail (it stands for Google email, you know) dot com.