I used a manual butter churn for the first time and made delicious butter. Yay! And it left behind something that resembles milk... It's lighter than the heavy cream that was churned into butter. It had warmed a bit during the churning process so I stuck it in the fridge. Can I use this for anything? Is it basically milk?
Technically, this is "buttermilk" - the milk left over from churning butter.
Of course, this is sweet buttermilk, so it won't really work for most recipes calling for buttermilk (they assume the cultured version, which is acidic, and has that purpose in recipes).
For sweet buttermilk, you can just drink it. You can use it in cooking, much like milk - perhaps closer to skimmed milk, since much of the fat (butter) has been removed. You could maybe make cheese with it, if you want, though it will be a lower fat cheese.
You can take your chances making cultured buttermilk the old fashioned way (leave it until it's "soured"), if you don't mind the risk. You can culture your own buttermilk - I've had success taking regular store bought buttermilk, and adding a glug of that to milk to culture it (or pouring milk into the freshly emptied bottle and letting it sit), or else you can find the actual culture used (look for places with cheesemaking cultures as a starting point and refine your search from there). The last may only be worthwhile if you're planning to make buttermilk regularly, otherwise it's a lot of effort for a one-time product.