Placeholder text can be a good place to start to introduce what is expected of the user before they begin interacting with a field.
Once they have focused on a field, subtext could appear beneath the field which elaborates upon the previous placeholder text–providing more context for the user, without the need for a tooltip.
Tooltips can be useful in scenarios where a user has many fields to fill out and–for some reason–these fields can’t be spread out across more screens. Then, a tooltip (usually a ?) can be used to ‘show’ further information.
Bare in mind: if the information is important to the user, it should never be hidden behind a tooltip.