What happens to my resources and gold at the end of a Reign?

It depends on the resource:

• The 4 base resources (Wood, Stone, Iron, Food), plus Meat and Resolve, reset. No point hoarding them late in a season: spend them before the end on useful research or troops, or they're gone.
• Gold, on the other hand, is permanent: it never resets and carries into the next Reign. That's your real store of value.

A good end-of-Reign reflex: empty your base resources into whatever still serves you (combat, campaign objectives) rather than letting them vanish at the reset. But guard your Gold: you'll be able to invest it next Reign in permanent purchases (Relics, Cores, keys at the Market, the third builder…). Don't burn it catching up on disposable resources.