What is your background?
I'll assume some maths.
I can try without maths if you need it.
dB is a log scale.
The markings on a ruler are "linear". Each 1cm you move along a ruler means the distance is greater by the same amount.
2cm is twice as far as 1cm
3 cm is twice as far as 1cm
x cm is x times as far as 1cm.
d = xcm This is a linear relationship.
not quadratic, d = x^2, not cubic, d = x^3.
d = xcm is a linear equation.
Bels, as in deci-Bels is a log scale. Distances along the dB scale are log(quantity)
As they explain each Bel, 10 dBels makes it times 2 loudness. 1 2 4 8 , not linear 1 2 3 4
Earthquakes are also ranked on a log scale. a 7 is much bigger than a 6. (Again, not a linear scale)
Hope this helps.
Comment if you need it explained differently.
The linear meaning you found is to do with sequences of things, not multiple paths. It's different.