Gold is the super currency in the game that speeds things up and gets you special items you can't earn or generate in the game.  You get 100 gold each time you level up, you can get gold flowers from the dog (read the link below for details on growing wildflowers), or you can buy it.  Two things to consider are use gold on things that you can't accomplish with time or hard work (don't build a cow with gold because you are short on cash, but do invest in the external barn) and when you buy or spend gold, do it in sales.  The "Super Sale" package gives almost 6x the gold of a base package.  Gold goes twice as far if you upgrade your fertile fields or external barn at 50% off.