Multiplier stacking occurs in two chief forms:
1.Flat stacking(adding): each modifier from each sources is added to total sum A+B+C=S.
2.Multiplicative stacking: each modifier is applied individually(usually with percents).

item A gives 7% fire resistance
Item B gives 11% fire resistance.
Item C gives 5% fire resistance
Skill D gives 3% fire resistance
With flat stacking:7+11+5+3=26% fire resistance.
With multiplicative stacking:1-( 0.93*0.89*0.95*0.97)~=0.2372 or 23.7% fire resistance.

Advantages of multiplier stacking:More balanced and gives small modifiers power at every step.
Flat stacking is easier to comprehend for average gamer.
The key difference, is multiplier stacking doesn't need to have a cap, while flat stacking can easily reach 100% or some percentage that makes modifiers overpowered, e.g. 10*10%=100% vs (10%^10) 0.9^10(65%).