How to understand this formula.?
Ferrite Rings Core 63x50x25 58.6uH@10kHz what does this mean by the
way..?

Well, that information is directly from the mouser website and, it is at odds with the data sheet. The Mouser website is appearing to say this: -

Outer diameter = 63 mm (data sheet states 67.3 mm)
Inner diameter = 50 mm (data sheet states 46.5 mm)
Width = 25 mm (data sheet states 28.6 mm)
Inductance 58.6 μH (data sheet states 59 μH)


So, it's unclear where Mouser pulled this information from. However, further down the Mouser page they do state things that match the data sheet. So, my advice is ignore that earlier statement and stick with what the data sheet tells you.

Specified value: 44 µH ≤ AL ≤ 85 µH

That information is telling you what the range of values the inductance may be for one turn of wire. In other words it's their guarantee that if you bought a hundred units and tested each unit you might get a range of inductance that approached those limits.

Setting values: Ieff x N = 40 mA f = 10 kHz

Here, they are telling you how the test can be done. They talk about Ieff and N and the product of those must be equal to or less than 40 mA. In normal-speak, they are saying that there is a limit on the ampere-turns used when measuring inductance. They say this because it's quite tricky to measure inductance with just one turn and you may choose to wind (say) ten turns. If you wind ten turns then make sure your test current is less than 4 mA and, in all probability that is the peak current of the 10 kHz sinewave you apply. The ampere-turn limit is to avoid transformer core saturation giving an inductance value that is falsely low.
Bear also in mind that 10 turns will produce a nominal inductance value of 100 x 59 μH because inductance is proportional to turns squared.