Compression runs entirely in your browser. Your image is never uploaded.
Compress to a size, not a guess
Most compressors give you a quality slider and leave you to drag it until the file looks small enough. If you have a hard limit, like a 2 MB upload cap, a 200 KB email attachment, or a forum's avatar size, that is the wrong tool. Tell this one the size you need and it does the searching: it tries different quality levels to find the highest one that still fits, and only shrinks the pixel dimensions if quality alone cannot get you there.
JPEG or WebP?
JPEG works everywhere. WebP usually reaches the same quality at a noticeably smaller size and is supported by every current browser, so prefer it unless you need maximum compatibility. PNG is lossless and cannot be tuned to a size, so PNG inputs are re-encoded to your chosen format.