Gold is the currency used in Dota to purchase items. In most modes, except random and alliance pick modes, you start with 603 gold if you choose your hero (randoming a hero will provide you with more starting gold). The game design and balance is based around this starting number. If you want to make sure you get this same number when playing with less than 5 players, set the unused player slots to computer. Every 0.8 seconds each player is given 1 gold (2 in easy mode) this results in 5 gold every 4 seconds.