Answer: Hi, isolines refer to lines where some quantity maintains constant.
For example, an "istoremal line" is a line where the temperature is constant, an "isobaric line" is a line where the pressure is constant.
Where constant means that, in the two examples i give you, in all the line the temperature or the pressure has the same value.
A thing that you usually can see in a whether map, is that the isolines are "closed" curves. And other thing, maybe more obvious, is that a line cant pass over other line (because this will mean that a point in the space has 2 different values of something).
