Glad you asked that. You see it all the time on butter packages, yet for most of us the phrase has virtually no meaning. “Sweat cream”? What on Earth can that be? It’s not sweetened cream if that was your guess. Rather it refers to cream that has not gone sour, i.e. it is “sweet”.
Ugh. Sour cream butter? Who would ever eat that? The answer is that before the advent of refrigerated trucks, pretty much everyone did. You see, once upon a time, if people wanted butter they had to make it themselves. They’d milk their cows into pails, pour the pails into tall milk jugs and wait for the cream to rise to the top. That cream would then be skimmed and put into a churn to make butter (the skim milk would then be fed to the pigs).
These steps didn’t always occur in rapid succession. Sometimes the separating milk was allowed to sit for a while before the butter-making commenced. In that case, lactic acid bacteria would move in and begin the “souring” process. It wasn’t harmful (usually) and in many cases tasted rather good, so nobody thought too much about it.
This practice of letting cream sour was taken to what one might call an extreme in the early days of commercial creameries, which made butter on an industrial scale. Often, these businesses lacked the size dairy herds they needed to produce large quantities of butter, so, they’d send a truck around to nearby farms to pick up jugs of whole milk that farmers had to spare.
The pickups weren’t always terribly punctual. Sometimes the whole milk would sit in the barn not for hours but for days, by which time it wasn’t dangerous (or at least not usually), but it was completely and totally sour. This wasn’t a problem for the creamery, which would simply pasteurize the milk to kill off any bacteria (friendly or otherwise), then “neutralize” the acid in it by adding an alkaline of some kind. Say, a little lye. Not enough to make it dangerous of course, though the souring and subsequent neutralizing process did have the effect of making the resulting butter taste a little less than farm fresh.
With the advent of refrigerated trucks in the dairy business (around the close of World War II), those old-timey practices began to change. Dairies and creameries could, for the first time, guarantee their customers that the milk their butter was made from was never allowed to sour. Thus the advertisement on the side of the box calling out “sweet cream” butter.
Not every creamery went entirely in this direction. Some, though they employed refrigeration from cow to counter, preferred a mildly soured cream for its complexiy of flavor. So, they’d add a little lactic acid culture to it prior to churning to make their butter more interesting. This is what “cultured” butter is and it’s still found here and there in the States (though it’s the norm in Europe).