Checking a file's DPI answers one question: what does the tag inside this JPEG or PNG say, and where does it come from.
What gets read
A JPEG can store its resolution in two places at once, the JFIF header and the EXIF data, and a PNG stores it in a chunk called pHYs. PIXLFLW reads whichever records are present and shows the value together with the one it came from, so you know exactly what a print shop or portal will see when it opens the same file.
When two programs disagree
If a JFIF record and an EXIF record in the same JPEG hold different numbers, different programs can report different DPI values for the file you thought was one consistent thing. That is not a bug in this tool or the other one; it is the file itself carrying two answers.
No DPI tag at all
Some files, especially ones straight off a phone or saved from a messaging app, carry no DPI record whatsoever. This tool says so directly instead of guessing at a number, because a made up value would be worse than admitting there is nothing to read.
Nothing is uploaded
The file is read entirely in your browser. It never reaches a server, which you can check yourself by opening your browser's developer tools and watching the network tab while you drop a photo in.
Need to set a specific DPI rather than just read it? Use the change DPI tool instead.