Drop PNGs — one or forty. Each is written as JPEG at matched quality. A PNG with transparent areas waits for you to choose what they become, because JPEG cannot hold transparency.
What happens to the PNG
The PNG is decoded in this tab and written as a JPEG. The JPEG quality is not a fixed number: the converter searches for the lowest setting whose result still measures visually identical to your PNG (multi-scale structural similarity of 0.990 or better, on a 512-pixel reference), so the file is as small as it can be without visible loss by that measure.
JPEG has no transparency. If your PNG has transparent areas, nothing is written until you choose a background — white, light grey or black — in the Transparency panel. The choice is shown on the stage before you save.
When JPG is the wrong answer
Screenshots, logos and diagrams are flat colour with hard edges, which is what JPEG handles worst: edges soften and ring, and the file is often larger than the PNG. The converter detects these pictures and says so. It still writes the JPG you asked for; switch the target to Best per picture to see what it would keep instead.
A JPG cannot be turned back into the exact PNG. Keep the original if you may need it again.