Bots are essentially passive power-ups that you can equip before starting a game. The do such things as increase your score, make ships move faster, give you extra lives, and make coin and XP payouts larger. There are 14 bots in total to obtain. You start with two already unlocked, but the other 12 must be purchased with coins. Coins are collected while playing the game, unfortunately at a rather low frequency, in two ways:
1) Landing the ships that are towing coins behind them
2) Tapping the large spinning coins that periodically fly across the screen
Make sure to purchase the More Coins Bot first and always have it equipped. This will make the painstaking process of earning coins faster, but not much. In total, it will cost 221,500 coins to purchase all the bots. With the More Coins Bot equipped, you will earn 100-200 coins per minute, depending on the game mode and luck, which equates to 35 to 75 hours of solid playtime just to earn the required amount of coins to purchase all the bots.