Here is a solid write-up on what this means and its pros/cons based on standard technical documentation: Key Characteristics of Solid Archives
Best for archival purposes, distributing software, or backing up folders with many similar files where maximum compression is needed. DELUXE.rar
Updating or adding files to an existing solid archive is slower because the entire archive often needs to be re-processed. Here is a solid write-up on what this
This helps me guide you on the best software or settings to use. Don't Use ZIP, Use RAR - Coding Horror Use RAR - Coding Horror