Someone who makes a wise judgement about something that has previously proven difficult to resolve.
This phrase doubtless alludes to the Biblical character Daniel, who was attributed with having fine powers of judgement. In Daniel 5:14 (King James Version) we have:
I have even heard of thee, that the spirit of the gods is in thee, and that light and understanding and excellent wisdom is found in thee.
The first use of the phrase as we now know it is from Shakespeare’s The Merchant of Venice, 1596:
A Daniel come to judgment! yea, a Daniel!
O wise young judge, how I do honour thee!