The idea of the product backlog is that it's prioritized and estimated to be healthy. However, something possibly over looked is that the top (a sprint or more's worth of work) needs to be understood by the product owner well enough that they can make important decisions about it!

Take a look at this diagram:

If this was your backlog:

• Green: Backlog items that the business can answer questions on concisely. Maybe 1 day turnaround on answering a specific question like "what text should be here" or "what will the user experience when they click this button"
• Yellow: Backlog items that the business has a lot of information about but probably isn't ready to have in a sprint yet. They're probably still gathering requirements from stakeholders and users.
• Orange: These are items that are probably really fuzzy to anyone. They're more of a wish list of things and aren't really fleshed out more than a high level 10,000 foot view or description.
• Red: Most likely these are something that someone asked for, but very likely brings no real business value and have been pushed farther and farther down.

The significance of the dark bar was just me accidentally drawing a thicker line, however... it is also probably a good place say "anything below here isn't fleshed out enough to become part of a sprint yet". This has to be your product owner's responsibility.