Fluid circulation around a closed curve

[influx]

I know that the circulation is defined as the counter clock wise integral around the closed curve of the flow velocity component along the curve but what is its meaning in real life? I mean what does circulation actually refer to in real life? Also could someone explain the above image? What is the relationship between stream lines and the closed curve? Sorry for the lack of working out but it's just an issue that is baffling me at the moment and I'm unsure of how to proceed

 

[Stefan2015]

I feel that this is related to Bernoulli's principle. For example check out this video:

Bernoulli's principle basically states that a higher velocity of fluid means lower pressure and lower velocity means higher pressure.

 

[boneh3ad]

First of all, that YouTube video posted by @Stefan2015 is awful and is filled with half-truths.

Anyway, @influx, the circulation in this sense essentially means that if you were to subtract off the free stream component of the velocity, you'd be left with a disturbance flow that tends to circulate the wing. This is a feature of lift-generating airfoils and can even be used to calculate lift (the Kutta-Joukowski theorem).

There isn't really an important relationship between the streamlines and the closed curve. The closed curve is arbitrary. The author of your book used the streamlines to show that the flow over the airfoil is still moving entirely in one direction yet has a nonzero circulation.