Beginner tips are tips that anyone can use. If you mess up while following a beginner tip, simply delete everything you've done and begin. Beginner tips pose no risk to your template.
Intermediate tips require a little more skill. Some of them involve making changes to the html of your template. One saved misstep while working with the html of your template can bring an undesirable effect to your blog. It is super important that you go slow and pay close attention when following an intermediate tip. You'll need to download a copy of your template if the tip calls for making changes to the HTML of your template. I recommend creating a practice blog to experiment with.
Advanced tips are for users who have successfully implemented beginner and intermediate tips with ease. They can be very difficult and most require several attempts before getting them right. Serious problems can result if a mistake is made while using an advanced tip, so try them on a practice blog first. As always, make sure to download a copy of your template before working on your real blog.