what exactly is maintenance?

This app really doesn't consider toning, however, if you want to build more muscle, you could set your weight goal to higher, and attempt to gain muscle weight.

I've got the weight loss thing down... I know what it's all about and I know what works for me.

But what exactly is maintenance? I assume maintaining your current weight, but how does that tie into getting stronger and toning and body fat % and so on?

I'm not overly focused on what the scale says, but I do know from past experience that if I get much lighter/thinner I start start to look thin, as opposed to looking fit and healthy. But I've still got some toning to do, and I know my BF % is still higher than I'd like it to be.

I'm just trying to figure out if I need to change my calories/macro goals, my training, etc.

Maintenance is the caloric point at which you do not gain or lose weight. You will hear very conflicting feedback as to whether or not recomping (eating at maintenance while training) is effective or not and there are a few factors you should consider:

It's possible that you will need to consider bulk/cut cycles, but there's no harm in giving recomping a shot to see if it works. It worked very well for me but I had training experience (muscle memory) on my side from about 10 years back, and I firmly beleive that this is a factor.

Without knowing your current calories/macros and your training procedures it would be hard for to recommend possible changes.

Maintenance is the caloric point at which you do not gain or lose weight.

I assume by weight you mean what the scale says... overall mass? If so, that implies that while in maintenance it's possible to change body density/volume, correct?

Yes but it depends on a number of factors. For example, and this is anecdote:
1) Skinny-fat people.
2) People who have very little training experience with weights
3) People who used to be in shape and became skinny-fat through not training (me one year ago).

These are examples of people who I would consider good candidates for recomping. It worked wonders for me.
Now, take a bodybuilder who has been training for 5 years and already has a well developed physique-- they'd probably not pursue recomping as an option.

You should also do your own research on this IMO. I would recommend checking the following sites to see what they have on recomping (recomping by the way is the term for eating at maintenance or eating with cyclical intake such that it balances at maintenance over the course of days-- I believe Martin may have something on this at leangains but I'm not 100% sure).

Check out the articles on bodyrecomposition.com and leangains.com, see what you can dig up there as it should be good reading.