That's a slightly confusing way to describe it I think
lerp(a, b, x) = a + (b - a) * x
lerp gives you the value x% of the way from a to b. So if x is 0, it gives a, if x is 1 (100%), it gives b, and if x is 0.25 (25%), x gives the number 25% of the way from a to b, etc. For example:
lerp(60, 100, 0.25)
= 60 + (100 - 60) * 0.25
= 60 + 40 * 0.25
= 60 + 10
= 70, the number a quarter of the way from 60 to 100.
Lerp is incredibly handy if you get to know it well and is used all over many games (and several times inside Construct itself too).