"That about covers it" means "that's pretty much everything there is to say about it", or simply "that almost covers it".
Edit: now that you have added a bounty, it's obvious that the above explanation doesn't help you that much. So I went out and dug up that quote in context:
As you can see, each filtering or projection simply returns a new collection for your use. This convenient notation can be used to get the most out of a single set of data. Note that you can combine filtering and projection operations. That about covers it for aspects of OGNL that are commonly used in Struts 2. In the next section, we’ll cover some of the advanced features that might help you out in a pinch, but, still, we recommend keeping it simple unless you have no choice.
As we can see from the context, "that about covers it" can be simply replaced with "so much". As in, "So much for the most common aspects of OGNL used in Struts 2". Or even: "These are the most common aspects of OGNL used in Struts 2".
What the author is saying is: "In this chapter here, we have covered some common, simple stuff. Chances are, that's all you will ever need. Yes, there is also more advanced, complicated stuff — and in fact, we will cover it in the next chapter, so do read on if you want to know more — but still, the simple stuff mentioned above will be probably enough for most people in their everyday work. Keep it simple!"