While there isn’t just one slider that does this, the closest one is the Exposure slider (found in the Develop module’s Basic panel). It doesn’t cover the darkest areas (blacks) or the brightest (whites), but it covers the all-important midtone range (and then some). So, when it comes to exposure, it has the biggest overall effect on your image. In earlier versions of Lightroom, there wasn’t an Exposure slider. Instead, this slider was called “Brightness,” and it controlled the overall brightness (although the math has been updated since it was Brightness). But, knowing that old name helps give you some idea of what this slider mostly does—it controls the brightness. Drag it to the right, your image gets brighter; drag it to the left, it gets darker.