It's certainly an intuitive-enough game but I frequently encounter icons that I can't make sense of. Is there some in-game manual that I'm missing? Any way to figure out what the various icons mean?

In particular, when picking my deck, I have "min 25/25" right next to "max 125/125." What is actually being counted with the 125/125?

It's supply limit. Every unit card costs certain amount of supply when you put it in your deck. You can see this number in bottom right corner of the card. Special cards, like artifacts and trinkets do not cost supply (only in Thronebreaker, not in Gwent). There are few buildings that increase your maximum supply and decrease the amount of supply points units require.