Uncompressed audio is audio without any compression applied to it. In other words it's the same quality and size as the original file.

Lossless audio compresses audio without losing any information or degradation in quality.

Lossy audio attempts to remove as much 'irrelevant' data as possible from the original to produce a file smaller than the original that sounds almost identical. It results in a lower bitrate and file size than lossless or uncompressed audio.