A code query can reference some code elements through string like in from m in Methods where m.IsUsing("System.String").
The resolving step aims at resolving these code elements from a code-base or a compare-context.
The reason for having IQueryCompiledUnresolved is performance. It takes much more time to compile a query than to resolve it to a code-base.
With IQueryCompiledUnresolved, the result of a compilation can be re-used on several code-base if needed.

Reference

IQueryCompiledUnresolved Members
NDepend.CodeQuery Namespace