Dic: an idea (as a general understanding based on knowledge)

• Could you give me an idea of how bad his injuries are? 
• You must have some idea (=have at least a little knowledge) of what happened to the money.
• Don't worry if you don't understand it right now - you'll get the idea (=begin to understand or be able to do something) .

■ idea [U & C] △a general understanding of something, based on some knowledge about it.

• She doesn't have any idea where they've gone. [have no idea/not have any idea]
• Can you give me a rough idea of how much the repairs will cost? [a general/rough idea(=a not very exact idea)]
• I don't have the faintest idea what to get Rachel for her birthday. [not have the faintest/slightest/foggiest idea(spoken)]

.... LDOCE