In my opinion and from seeing the type of questions that are closed by the community, the site is for DIY in and around the home.
So questions about DIY car repair are off topic, DIY electronics seems to be boarder line and might get answered (depending on what your working on). DIY furniture building can go either way, but toy making is off topic (no elves here, sorry Santa). DIY roof repair and deck building are defiantly on topic, and basic lawn and garden care are also OK.
If your not sure about a question, ask it. The worst that will happen is the question will be closed, and it will help further define the site.
The best way to see what is on/off-topic is to visit the site often (and contribute), you'll notice the types of questions that get good answers and the type that don't (or get closed). The site is defined more by the community than it is by the faq, so it's hard to definitively define exactly what the site is about.
Really it comes down to what the community is able to answer, and what knowledge those contributing to the site have. As the name states you'll notice that questions about building/repairing/improving the home get great answers, but fringe questions teetering on the edge of off-topicness will not. So again the best way to figure out what the site is about is to hang around for awhile, and get to know the community.