What changes when you convert PNG to JPG
This conversion is usually about size. PNG is lossless, which is the right choice for screenshots, logos and flat graphics, and the wrong one for photographs — a photograph saved as PNG can easily be ten times larger than the same image as JPEG with no visible difference. Converting is the single biggest saving available on that file.
The direction matters for what you lose. A photograph converts beautifully: JPEG was designed for continuous-tone imagery and handles it well. A screenshot of text converts badly, because JPEG's artefacts cluster around the hard edges that text is made of, and the result looks smeared at exactly the places you were trying to read. If the PNG is a screenshot, resist this conversion and compress the PNG instead.
Transparency does not survive. JPEG has no alpha channel, so transparent regions are composited over white. For a logo intended to sit on a coloured background, that produces a white box around it, which is almost never what you wanted. Keep logos and icons as PNG.
Metadata is dropped as a side effect of the canvas round-trip. EXIF, colour profiles and any embedded copyright field go with it. Losing the profile matters for images destined for print, where the difference between sRGB and a wider gamut is visible; for web use it is generally the desired outcome.
How much smaller depends entirely on the content. A photograph saved as PNG commonly drops by ninety percent or more with no visible difference, because JPEG is doing the job it was designed for. A flat graphic with few colours may drop by very little, or even grow — PNG's palette compression is already extremely efficient on that kind of image, and JPEG has nothing to exploit. The result panel shows the measured before and after, so you can see which case you are in before you replace the original.
Keep the original either way. A PNG converted to JPG cannot be converted back — the discarded information is gone, and re-saving the JPG as a PNG produces a large file containing the JPEG's artefacts. If the PNG is a master copy or a source asset, convert to a new file and leave the original alone.
The decode and re-encode both happen in your browser. No upload, no queue, and no third party holding a copy of an image you were only trying to shrink.
This page is the PNG-to-JPG case specifically. The general tool, with its full set of limitations and questions, is Compress image.