Starter packs are a feature available to all players at the cost of Tokens (purchased with real money) in order to give them an early boost in the game, hence the name "starter" packs. A starter pack will upgrade bins and fill them, unlock some token recipes, and give the player some game money. While all of these features can be either bought individually (token recipes) or bought with gold, starter packs also give the player bonuses that are otherwise impossible to get: bonus points to worker stats and more gamedays to store (9, 11 or 14 depending on the pack instead of 5 or 7 depending on first game date or account creation). This last feature is probably the most interesting of all since it can benefit you even if you are a high level player already.
Although they are called starter packs, they can be bought anytime. Only one starter pack can be bought unless you reset your game, in which case it is lost.
Bins will not be downgraded if they are already higher than the starter pack would upgrade, but they will be filled only to the amount specified in the pack.