To add to what dqhall said, the Israelites would gather those first and then in verse 14 "And you shall eat neither bread nor grain parched or fresh until this same day, until you have brought the offering of your God: it is a statute forever throughout your generations in all your dwellings." So there is a fast involved that they wouldn't eat bread until the offering had been made.
Although I'm wondering how this was done in practical terms. It would be a bit difficult for every person to pick up and travel to Jerusalem. Was it done with some kind of tribal or village representative that would take the community's offering?