This likely refers to the diameter of the entry holes in the junction box. A 20mm junction box would have entry holes that are 20mm in diameter.
Polycab is a well-known brand that manufactures electrical and cabling products. In this context, it indicates that the junction box is produced or branded by Polycab.
A "One-way" junction box typically means that it has four cable entry points, allowing you to connect or branch four separate electrical circuits within the same box.
This can be useful for creating electrical connections, splices, or distributing power to multiple locations.