What does solid archive mean?

Solid archives are RAR-archives that are compressed using a special algorithm that copies all the files to be compressed into a single file before actually packing them.
Solid archives significantly increase compression rates especially when many smaller, similar files are being compressed.

1.

You have several files, some are the same type.

2.

Choose this option in the dialog box

3.

WinRAR handles all files together as a single data stream. Small, similar files, text files in the example shown, result in a final output that is significantly smaller than it would otherwise be.