Linear, in it's most basic form, takes 3 arguments and looks like this: linear(progress, from, to) ;
progress = a number between 0 to 1 determines how much to blend the values 'from' and 'to'.
from - blend from this point (or number, or color, etc...)
to - blend to this point (or number, or color, etc...)
Linear blends 'from' and 'to' and returns a new value. Exactly how much it blends between the two is dictated by 'progress'.
In the following interactive example, linear is applied to a position property. You can see how the result is a point between the 'from' point and the 'start' point. Exactly where is determined by the 'progress'.
Linear(0, [0,0], [100,100])