PNG is great for screenshots and graphics with transparency, but its lossless compression makes files much larger than a photo really needs. Converting to JPG is the usual move when you're attaching an image to an email, uploading to a site with a strict size limit, or just want a smaller file for a shared photo. Because transparent areas get flattened onto a white background during conversion, expect a visible change if your source PNG has see-through regions. For fully opaque photos and screenshots, quality loss at reasonable JPG compression is minimal.
What happens to the transparent parts
PNG stores an alpha channel, a per-pixel record of how see-through that pixel is. JPEG has no such channel and no way to add one, so transparency has to go somewhere during conversion. It gets composited onto a solid white background. For a screenshot or an opaque photo that changes nothing at all, since every pixel was already fully opaque. For a logo with a cut-out background, the result is that logo on a white card, which looks wrong the moment you place it on a coloured page.
That single behaviour decides whether this conversion is right for your file. If the transparency matters, WebP keeps the alpha channel and gets you most of the size saving anyway.
Why the PNG was so much larger to begin with
PNG compresses losslessly, reconstructing your image bit for bit. It does that by predicting each pixel from its neighbours and compressing the differences, which works beautifully on flat colour and hard edges and poorly on photographic noise, where no two adjacent pixels agree. JPEG instead throws information away, discarding the fine detail human vision is least sensitive to.
This is why a photo saved as PNG can be five times the size of the JPEG that looks identical, and equally why a screenshot of a code editor converted to JPEG can come out looking worse than the original at a similar size. Photographs and gradients convert well. Text, line art, and flat blocks of colour with sharp boundaries are where JPEG's artefacts become visible as fuzzy haloes around the edges.