I know that ∫B.dl = μI... but what does the quantity ∫B.dl mean/represent?
This should be well explained in any EM book.
Anyways, you take a closed loop. B(r) is the field at any point on the loop. You find the line integral of B over the closed loop and that happens to be equal to uIenc.
If you knew vector calculus, you'd be able to derive this from the (differential) Maxwell equation.
This is the magnetism equivalent of the Guass law of electrostatics. You can use it to obtain the magnetic field around symmetric current distributions, such as a wire.
It's called the circulation of the (magnetic) field. It has no immediate physical meaning, but there are powerful theorems of vector calculus that relate this quantity to a surface integral ot the curl of the field over any surface subtended over the loop in the line integral. BTW, the Ampere's Law in the form you posted is only true for stationary fields.
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