Some teams new to agile start with a new project and need to estimate the whole thing with little info, perhaps during a "sprint 0". Those teams are forced to deal with ambiguity right from the start. From then on they accept that they'll have to estimate fuzzy stories. This helps them accept that stories will change, both the details of individual stories as well as what stories make up the backlog. Sometimes revealed detail changes an estimate (up or down). Sometimes it doesn't. But that's life and we can deal with that quite well on an agile project. More on that in a moment.
Other teams I work with already have a project well underway. They have a product in maintenance when they go agile. They've got lots of small and well understood stories. Or if they aren't well understood, they can get that way quickly. The 1st stories they estimate are detailed and certain. But a day will come when the team is forced to estimate some new release or some new product with less than full information. That can be frightening. As these estimates are recorded, we can note that the estimates were made with less info and certainty, but that seldom provides much comfort to this type of team.
However, what you'll find (on a healthy team) is that:
- Large estimates on the larger stories tend to drive the Product Owner to decompose those behemoths and then ask for new estimates.
- The team is allowed and even encouraged to revise the estimates (up or down) as more information is revealed.
- Many of the estimates will not need to be revised. Any additional detail or certainty often doesn't affect the estimate after all.
- Consistency is good, but accuracy and precision are not necessary. Wrong estimates have an offsetting impact on velocity. That's the beauty of using relative estimates and the empirically measured velocity to compute a release end-date -- an incorrect increase in one gives an offsetting decrease in the other.
- There will be details that are overlooked. Developers tend to be optimistic with estimates. But that too will soon be taken into account in the velocity (unless you implement all the well understood stories 1st and always save the others for later). (Do note, however, that the shorter your release is the shorter your iterations need to be in order for a change in velocity to be noticed in time.)
- And management will heed the warning signs when presented with the estimates and velocity and not pressure the team into short-cutting craftsmanship, quality or sustainable pace. (Remember that I said /healthy/ team.)
Seeing is believing
On the second type of team this learning happens slowly. You can have a conversation around these things to try to speed up the learning or to provide some comfort. It's certainly worth trying. But don't be surprised if your convincing arguments are met with disbelief.