What is the meaning of unambiguous?

Well, something is said to be 'ambiguous', when its meaning is unclear and open to more than one interpretation, as in the following

"The prime minister made an ambiguous statement regarding his intentions for health policy reform."

Thus "unambiguous" is the exact opposite (ie clear) of 'ambiguous' as in:

"The president was unambiguous in his statement that the destruction of the twin towers presented a real threat to US homeland security"