What does mean "fl oz"?
"fl oz" means liquid ounces (1 fl oz ~= 30 ml), as opposed to solid ounces.
How are they calculated the 30:1 ratio?
By ratioing 128:4.26, it is 30.05:1, so 30:1 is a good approximation
What does mean "2-cycle oil"?
It is oil for 2-cycle piston engines
from where these numbers have came?
Probably some design specs/limitations. You have to ask them if you want the details.
What does it mean?
That when you refuel, for every 30 parts of fuel you put in the tank, you need 1 part of oil for correct lubrication of the engine.