Offerings can be used to alter the trial from the very start. Some offerings can increase the amount of Bloodpoints you can gain from performing certain actions, or increase the chance of visiting a certain map. Some offerings can even give you the chance to start a trial alone, or with other survivors in close proximity.
Killers also have access to their own set of offerings which can hinder the chance of survivors escaping, or increase points earned from their own actions.
You can find out more information about each offering by hovering your mouse over their respective icons.