What ID3 tags are
An MP3 file is a sequence of audio frames with no place to record who made the recording. ID3 is the convention that solves this by attaching a metadata block to the file, and ID3v2 puts that block at the front, before the audio. Inside it are individually identified frames, each holding one piece of information under a four character identifier.
The identifiers look cryptic but they are consistent. TIT2 is the title, TPE1 is the artist, TALB is the album, TPE2 is the album artist, TYER is the year, TCON is the genre, TRCK is the track number, COMM is a comment, and APIC is attached picture, which is where cover art lives.
This tool reads those frames from the file you open, shows them in a form, and writes your edits back into a new copy of the file. The audio frames themselves are untouched, so tagging never re-encodes and never affects sound quality.
The fields this tool handles
The editor covers title, artist, album, album artist, year, genre, track number and comment, plus cover art. The album artist field is worth a note because it is the one that keeps a compilation together in most music libraries: the per-track artist varies, the album artist stays constant, and software groups on the latter.
Reading is tolerant of how the tags were written. Text frames can be stored in Latin-1, in UTF-16 with or without a byte order mark, or in UTF-8, and the reader decodes all of those, which is why a file tagged years ago by other software with non-English text still shows correctly rather than as mojibake. Malformed or missing tags produce empty fields rather than an error.
Fields left empty are simply not written. The writer sets only the frames that have a value, so clearing a box does not guarantee that an existing value is erased in every player that reads the result.
Cover art
Cover art is stored inside the MP3 as an embedded image in an APIC frame, marked with a picture type. This tool writes type 3, which is the front cover, the type that music players display. If the file you open already has art, it is shown and kept unless you replace or remove it.
You can replace the art with a JPG or PNG from your device. Bear in mind that the image is embedded in every copy of the file, so a 4 MB photograph adds 4 MB to the MP3. Something in the region of 500 x 500 to 1000 x 1000 pixels as a JPEG of a few hundred kilobytes is a sensible target for a cover that looks right on a phone without inflating the file.
Remove cover clears the art from the saved copy. That is the way to shrink a file that somebody tagged with an oversized image, and the way to strip artwork that came from a source you would rather not carry.
A worked example: fixing an album of downloaded files
Open a 9 MB MP3 that shows up in your library as Unknown Artist. The form fills in with whatever tags exist. Set the artist, set the album, set the album artist to the same value across all tracks of the album, set the track number, and add a 300 KB JPEG cover.
Saving produces a download named with an edited- prefix in front of the original file name. Your source file is not modified in place, because a web page cannot write back to a file on your disk. You end up with a new file alongside the old one, and you replace the original yourself.
Repeat per track. There is no batch tagging, no automatic numbering across a folder, and no lookup of album information from an online database, so a twelve track album is twelve passes through the form. For a large library, dedicated tagging software with folder-level operations will be faster.
Limits worth knowing
This tool handles MP3 only. Files are accepted by the .mp3 extension or the MPEG audio type, and other audio formats are rejected. M4A, FLAC and OGG use entirely different metadata systems, and none of them are supported here.
The editing surface is the set of fields listed above. Lyrics, composer, publisher, BPM, disc number, compilation flags, ReplayGain values and custom user-defined frames are neither shown nor written. If a file carries frames the editor does not display, treat their survival as unverified, and keep the original file until you have confirmed the result in your player.
Everything happens in the page: the file is read into memory, the new tag block is assembled locally, and the result is offered as a download. Nothing is uploaded, and there is no account or history.