What does 'hard' 'medium' and 'soft' mean?

In terms of strings, I think they're usually the different gauges of them

high, medium and low tension respectively. You probably want "hard" strings on your little viola (could say more definitely if you post the vibrating string length (nut to bridge distance)).
Sometimes you see the strings labelled in German: "stark", "mittel" and "weich": high, medium, low tension respectively.

Thank you DiscoPants. I think you did say that before but I couldn't find the info
I'll measure the string length later today.