What does the expression 'as the crow flies' refer to?

As the crow flies (idiom): 
English Meaning - Used to say that a distance is measured in a straight line between two points or places. 
Bangla Meaning - সোজাসুজি/ সরাসরি দূরত্ব। 

Example Sentence - The river, as the crow flies, is just a few kilometers from here, but we'll need to drive around the canyon to reach it.

Some other phrases/idioms, 
As the case may be (phrase) - Whichever one of the stated possibilities is true. 
As the actress said to the bishop (idiom) - Used to show that someone has said something that could have another meaning connected to sex. 
As to (idiom) - About. 
As such (phrase) - In the true or exact meaning of the word or phrase. 

Source: Cambridge Dictionary.